In the Mecca Café, one of Seattle's most magnificently grungy diners, August Wilson is trying to communicate the rush of adrenaline that surges through mind and body when a new work is completed. Just hours earlier, in his basement study across town, he had finished the final draft of his latest play, Gem of the Ocean, set in Pittsburgh in 1904. The main characters include a 287-year-old matriarch named Ester, who has already cropped up in two of Wilson's plays, an escaped slave called Solly, who scrapes dog shit from the city pavements not for the sake of public hygiene but, Wilson explains, because "back then it was used in tanning hides", and Citizen Barlow, who is unsuccessfully trying to claim the citizenship promised in the 1865 proclamation on the emancipation of America's slaves.

"It's all structured around the conflict with the law, how the law is a weapon to oppress people," Wilson says. "Blacks wanted to leave the south and go north to seek their fortune, but the south was trying to hold on to its cheap labour and wouldn't let them go. Despite emancipation, you essentially have a situation that is like slavery."

Acting out great chunks of dialogue in the café, Wilson illustrates the various plot twists, including a journey on a magic paper boat to a city of bones in the Atlantic where slave ships run aground - a kind of Bermuda Triangle of African-American enchainment. In the final scene, Solly, pushed to the brink by thwarted hopes, tries to wreak revenge by burning down the local mill and the municipal jail: "That last scene has been running around in my head for over a year," Wilson says. "I've known how it ended, so it was simply a matter of writing it. Now I look at the script and there is something on all those pages that used to be blank. It's like you've just given birth. It's painful but it's a joyous process."

When Gem of the Ocean premieres in Chicago next April, it will form the penultimate chapter of August Wilson's decade-by-decade chronicle of African-American history in the 20th century, a grand theatrical cycle that has established Wilson, according to Time magazine, "as the richest theatrical voice to emerge in the US since Tennessee Williams and Arthur Miller". So far, instalments have included Fences, a towering family drama of generational conflict set in the 1950s; Ma Rainey's Black Bottom, a blues-infused story of session musicians in the 1920s; and King Hedley II, a tale of criminal survival on the Pittsburgh fringes set in the 1980s, which had its UK premiere at the Tricycle Theatre in London this week.

What is perhaps most remarkable about Wilson's work is that it has translated this uncompromising picture of disenfranchised African-Americans, profoundly influenced by the Black Power movement and the radicalism of the 1960s ("the kiln in which I was fired", he has said), into mainstream success and two Pulitzer prizes. The first black playwright to succeed on Broadway - with Ma Rainey in 1984 - since Lorraine Hansberry's A Raisin in the Sun 25 years previously, he has transformed himself into a one-man theatrical tradition. "He has created a body of material for black actors and black theatre people to look to," says black director and producer Lloyd Richards, who has nurtured Wilson's career. "There has been a lack of material on the library shelves on black theatre and a lack of theatre by blacks. All that is beginning to change. Having August's body of work in there leads people to an examination of theatre and an examination of themselves in theatre."

Wilson has come under sustained attack by pundits - predominantly white - who accuse him of prescriptiveness and grandiosity: "This single-minded documentation of American racism is a worthy if familiar social agenda," claims the New Republic's Robert Brustein, Wilson's most vociferous critic, "and no enlightened person would deny its premise, but as an ongoing artistic programme it is monotonous, limited, locked in a perception of victimisation." In a characteristically defiant reply, Wilson has labelled Brustein "a sniper, a naysayer, and a cultural imperialist". Others counter that the undeniable dramatic breadth of Wilson's writing both sustains and transcends any political message: "It's August's language," says director Marion McClinton, who has staged Wilson's recent works, "the rhythm of hurt, of pain, of ecstasy, the rhythm of family sets him apart."

This is a sentiment that would certainly be echoed by the new generation of Wilson-influenced black dramatists, including British writers such as Tanika Gupta, Roy Williams and Kwame Kwei-Armah, whose new play opens at the National Theatre in London next year and was directly inspired by King Hedley II: "There is nobody in this country in my experience who can be a celebrated artist in mainstream culture who can remain, as August has said on a number of occasions, quintessentially 'the man that he was when he entered into manhood'," says Kwei-Armah. "August is unapologetic for the politics that he has now, which are the same politics that he had when he first listened to Malcolm X, to Martin Luther King, and when he first spoke with James Baldwin."

In a jaunty peaked cap, Wilson comes across as both thoughtful and ebullient, a good listener but also a great talker, overflowing with ideas. His conversation is peppered with quotations, including from his own work. "Sometimes I think he has a photographic memory," says Wilson's friend, the writer and director Rob Penney.

According to poet Chawley Williams, the source of this restless energy is his background in a family with a black mother and a white father, and with five brothers and sisters varying in appearance from white, in the case of his sister Linda Jean Kittel, to black in August's own. "In all the time I've known him, August has been a very driven person," says Williams. "His vitality is enormous because he continues to delve within himself and his surroundings to connect with who he is. There was a major dilemma within him to resolve his being bi-racial. August will tell you and everybody else that he has always lived in a black world and he has always identified that way, but if you listen to his art, you'll hear the bi-racism." Wilson denies this, insisting that he "never felt anything other than black", and his sister Linda Jean agrees that he "was not torn with the bi-racial issue at all".

Wilson believes that black communities were better off under segregation: "I can only speak about what I know - Pittsburgh, the Hill District 1940. There were nine drug stores all within five minutes walking distance of each other, there were three wallpaper and paint stores, there was a fish shop, whatever the community needed. If that had been allowed to continue to develop economically and culturally, separate from the mainstream, blacks would be in a much stronger position today. But that was destroyed, and I'm not sure if it wasn't purposeful." In many ways he believes the lot of black Americans has worsened: "There are approximately a million black men in jail," he says. "The assumption is that they are criminals, but a lot of them are in for drugs violations. Again the law is being used against you. When you have somebody in jail that means there is a woman without a husband, there are kids without fathers. So the break-up of the family is part of the continuation of public policies and practices going back to the emancipation proclamation."

Wilson hasn't been afraid to pique black opinion, particularly in his opposition to "colour-blind" casting in the theatre: "It's the idea of obliterating yourself. Take an all-black Death of a Salesman - that's not the way black folks deal with their problems, it's not their language, it's not their relationship from father to son. We have a different history, different necessities. A guy can't go round and be a travelling salesman in the 1940s and be black. He'd get lynched. What is the point of putting black folks on stage as if they didn't have any stories of their own to tell?"

Wilson lives in a rambling, turn-of-the-century house on Seattle's Capitol Hill with his third wife, the Colombian costume designer Constanza Romero and their five-year-old daughter Azula. Despite being one of the most successful playwrights in America, he has strenuously avoided being sucked into the mainstream, and has turned down offers to write screenplays about Muhammad Ali and Louis Armstrong. When he started writing plays, he had never seen any by Chekhov, Ibsen, Eugene O'Neill, Arthur Miller or Tennessee Williams. He still rarely goes to the theatre and has seen only a handful of Shakespeare productions. Creatively, his life is devoted to exploring the buried history of America: "Blacks in America want to forget about slavery - the stigma, the shame. If you can't be who you are, who can you be? How can you know what to do? We have our history. We have our book, and that is the blues."

August Wilson was born Frederick August Kittel on April 27 1945 in the African-American Hill District of Pittsburgh, Pennsylvania, the son of a white German baker and a black cleaner named Daisy Wilson. Among black members of the Hill community, Linda Jean Kittel says, this was rarely an issue: "I always found there was more sincere acceptance among the blacks." The Kittel family, however, was a salient illustration of American racial values, with white neighbours, after the family moved to the predominantly white suburban neighbourhood of Hazelwood in the late 1950s, often treating the dark and light-skinned children very differently: "My brother's pain was always my pain," says Linda Jean. "It pained me also to know that I would receive privilege where he didn't."

All the children gravitated towards their African-American heritage, encouraged by the fact that their father was, as Wilson recalls, "a sporadic presence", largely due to alcoholism. But it was mostly due to Daisy who, despite financial worries, worked hard to maintain stability in the family. "We were what you would call a poor family," says Wilson, "but we were rich in so many things. We did family things together. We always had dessert even if it was just Jell-O. So, I never knew I was poor." In 1958, Daisy married an African-American man named David Bedford, a promising young athlete who had turned to crime, killed a man during a bungled robbery and served 23 years in prison. He died in 1969.

August - known as Freddie - was an intelligent and inquisitive boy: "He had a lot of natural ability," says Linda Jean, "at 10 he wrote what I'd consider his first poem." Daisy hoped that her brightest child would become a lawyer. However, he was forced to leave the all-white Central Catholic High School by increasingly vicious prejudice: "Every single day he would receive a note on his desk saying 'Nigger go home'," his sister Linda Jean remembers. Freddie's enthusiasm for formal education was finally killed off at his new school, ironically, by a mistrustful African-American teacher. "We were given this writing assignment on a historical personage of our choice," he says. "I liked Napoleon and I went to the library and I read six or seven books and turned it in. The teacher said he wanted to see me about my paper, and he showed it to me marked with an A+ and an E. And he says to me, 'I'm going to give you one of these two grades, depending on whether you can prove you wrote this'. And I said, 'If I hadn't written it I wouldn't have put my name on it'. To him that was a smart answer, so he circles the E. I tore it up, threw it in the wastepaper basket and walked out of school."

For the rest of the year, Wilson hid his decision to drop out by leaving the house every morning as usual and spending his days in the Pittsburgh Public Library. When he was 16, he took a succession of menial jobs: dishwasher, gardener, delivery boy, cook. However, inspired by his continued reading in the "Negro Section" of the library, where he first encountered the work of Ralph Ellison, Langston Hughes and James Baldwin, he nurtured an ambition. "These books were a comfort," he says. "Just the idea that black people could write books. I wanted my book up there too, I used to dream about being part of the Harlem Renaissance."

After a year in the army, in 1965 he moved into the basement apartment of a rooming house on Bedford Avenue and, with $20 from his sister Freda, bought himself his first typewriter: "I didn't have enough for the tax, but the guy gave it to me anyway. Then I didn't have enough for bus fare and had to carry the damn thing up the hill."

Now calling himself August Wilson, he began writing poetry, occasionally sending the results off to literary magazines, but mostly reciting his work to anyone who would listen at impromptu readings in bars, restaurants, jazz clubs, even in the street. "Some people asked if I had heard of this young man who was walking around with a pipe stuck in his mouth reciting poetry in the street," remembers Chawley Williams, another Hill District poet. "A lot of restaurant patrons thought he was crazy. Not many black people were involved in poetry then."

With other black Pittsburgh poets such as Carl "Dingbat" Smith, Rob Penney and Nick Flournoy, Wilson and Williams formed the Center Avenue Poets Workshop, which put on events in various Pittsburgh venues, much in the manner of the Slam Poets of the 1980s: "Sometimes we would have drummers, conga and bongo players and we would have jazz for the intro," says Williams, "and then each poet would come in and do their own thing. Sometimes we had 'free-for-alls', where everyone would free-associate and say whatever connected to what they were hearing, straight out of their hearts." The poetry was rough, raw and colloquial, betraying influences such as Dylan Thomas, the Beat poets and the ideas of black radicals such as Marcus Garvey and often dealing with the poverty and drug problems that afflicted the Hill District community. Though most of Wilson's poetry has disappeared, a few verses such as this, "Theme One: The Variations", appeared in anthologies:

lost niggers breathe again holy
niggers breathing out power
where we lie spirited again
where life is ours in our ever
moving breathing the real
in the catechism of the spirit
teaching and believing the dance
the shape the stars the real
nigger hands the real nigger
voice where it screams in life

It was at this point that Wilson took his first decisive step towards the theatre. Rob Penney also had an interest in playwriting and in 1968 formed the Black Horizon Theatre Company with him. The primary inspiration had been the visceral, politically charged work of Amiri Baraka, in particular his Four Black Revolutionary Plays. Penney also contributed two of his own plays, entitled Deeds of Blackness and Dance of the Blues Dead. "I became a director because no one else wanted to do it," remembers Wilson. The learning curve, however, was steep: "August didn't know anything about doing theatre before this," says Penney. "He went to the library and learned about theatre and about directing. It was on-the-job training and he was an extremely good director." Wilson made his playwriting debut with Recycle in 1973. Of this and other early efforts, Penney says: "They were sort of avant-garde, experimental plays. He hadn't really found his voice. They were much more poetry than drama."

Recycle was interesting primarily for its autobiographical subject matter, it dealt with Wilson's first marriage in 1969 - to Brenda Burton, a member of the radical black Muslim group the Nation of Islam - which had broken down in 1972: "My first wife is a good woman, I still can't say nothing bad about her other than the fact that we had a difference on religion," he says. "She wanted someone who was a Muslim who shared those values. And I was like a heathen. I had to stay home on Sundays and watch the football game." They had one daughter, Sakina Ansari, born in 1970.

The first discernable indication of Wilson's power as a dramatist came in 1976, with The Homecoming, an early version of Ma Rainey's Black Bottom, in which Wilson examined the exploitation of black talent in the music business. Jitney centred on the Pittsburgh Gypsy Cabs who were willing to venture into black neighbourhoods, while Black Bart and the Sacred Hills, which began as a series of poems, was expanded into a play for the Penumbra Theatre in St Paul, Minnesota. It was during one of his trips to work on the production that Wilson met social worker Judy Oliver. In 1978, he moved permanently to St Paul and, three years later, she became his second wife. The marriage ended in 1990. There were no children.

In previous years, Wilson had submitted plays to the O'Neill Theater Centre in Waterford, Connecticut, only to be greeted with polite rejections. Beginning with Jitney, his writing had begun to undergo a revolutionary change: "I came across a quotation by Sekou Touré: 'Language describes the idea of the one who speaks it'. Once I discovered that, it was just a matter of letting people speak." This approach bore fruit, in 1982, with Ma Rainey: "We's the leftovers," says piano player Toledo. "The white man knows you just a leftover. 'Cause he the one who has done the eating and he know what he done ate."

This time, the supple characterisations and fluid dialogue made an altogether different impression on the O'Neill judges: "I read the piece and I found the characters were very alive to me," says Lloyd Richards, who was then artistic director, "I recognised them, I knew them from their words, I knew their faces almost. So I decided to include him in the national playwrights' conference." After the conference, Williams approached Wilson with an offer to produce Ma Rainey at the Yale Repertory Theatre in Connecticut, where it premiered in April 1984 before moving on to the Cort Theatre on Broadway. Reviews were mixed, with John Simon of the New York Post complaining of weak structure and "intermittent drama". However, it was the all-powerful Frank Rich of the New York Times who deemed the show a hit, and it duly picked up a New York Drama Critics' Award: "Mr Wilson's style is all his own," Rich concluded. "He has lighted a dramatic fuse that snakes and hisses through several anguished eras of American life. When the fuse reaches its explosive final destination, the audience is impaled by the impact."

By this time, Wilson had already written Fences, which premiered at Yale in 1985 and, after intensive revisions, on Broadway in 1987. The story of a former baseball hero who kills a man and spends 15 years in a penitentiary, it was inspired less by Wilson's stepfather David Bedford than by Pittsburgh boxing hero Charley Burley, who had lived across the road from the Kittels. "The strongest, most passionate American dramatic writing since Tennessee Williams," Clive Barnes wrote in the New York Post, pronouncing the production "one of the richest experiences I have ever had in the theatre". James Earl Jones picked up a Tony best actor award for his portrayal of Troy.

After Fences, Wilson remarked that each of his plays had dealt with a different decade. He continued the pattern with Joe Turner's Come and Gone (1988), about a Pittsburgh boarding house in 1911 still haunted by the ghosts of slavery. The Piano Lesson (1990), a piece about a young man named Boy Willie who returns to Pittsburgh in the Depression era to buy up the land his enslaved ancestors once worked, cemented Wilson's reputation by winning a second Pulitzer. He followed up with Two Trains Running (1992), about the threatened redevelopment of a Pittsburgh restaurant in the 1960s, Seven Guitars (1996), a story of buried shame and dreams of retribution told through the reunion of five friends, and rewrites of Jitney (1996) and King Hedley II (2000), which together present a harrowing picture of drug-ridden slum life in the 1980s that supports Wilson's central thesis that, despite the hopes of the 1960s and the civil rights movement, the lot of African-Americans has not improved throughout his century of plays: "When you look at black America, yes we have Colin Powell, we got Clarence Thomas and we got the head of Time Warner," he says. "That comes to about 10 people out of 35m. The majority live in dire poverty in housing projects without any avenues for participation in the society."

Now that Wilson's magnum opus is reaching its conclusion, the question arises of what he will do next: "I'm going to start over," he says, "I've got a couple of ideas for plays that didn't fit into my cycle. And I want to go back and do some of those." One sure indication that he could never retire, he says, had come that same morning when he finished Gem of the Ocean: "I immediately got depressed. I walked around totally disheartened, not knowing why. It's like you have temporarily lost your reason for living and the only thing to do is to start another. So, I will write down a title or a line of dialogue or an idea for a character or something. So, if someone asks me what I am doing, I can say: 'I'm working on my new play'."

Frederick August Kittel: Life at a glance

Born: April 27 1945, Pittsburgh, Pennsylvania.

Education: 1959 Central Catholic High School, Pittsburgh; Connelley Vocational School; '60 Gladstone High School.

Married: 1969 Brenda Burton (one daughter, Sakina Ansari '72) divorced '72; '81 Judy Oliver, divorced '90; '94- Constanza Romero (one daughter, Azula Carmen '97).

Dramatic Writings: 1973 Recycle; '76 The Homecoming; '77 Black Bart and the Sacred Hills; '82 Jitney; Ma Rainey's Black Bottom; '85 Fences; '88 Joe Turner's Come and Gone; '90 The Piano Lesson; '92 Two Trains Running; '96 Seven Guitars; '99 King Hedley; 2002/3 Gem of the Ocean.

Awards include: 1984-85 Best Play (Ma Rainey's Black Bottom) New York Drama Critics Circle; '87 Pulitzer Prize for drama (Fences); '88 Best Play (Joe Turner's Come and Gone) New York Drama Critics Circle; '90 Pulitzer Prize and Best Play (The Piano Lesson) New York Drama Critics Circle; '92 American Theatre Critics' Association Award (Two Trains Running); Bush and Guggenheim Foundation fellowships.

· King Hedley II, produced by the Birmingham Repertory Theatre, is at the Tricycle Theatre, Kilburn, until February 8. Box office 0207 328 1000