Bunhill Fields, the London cemetery where some of the most radical figures in history lie quietly side by side in unhallowed ground, will today be declared a Grade I park by the government, with separate listings for scores of its monuments.

The cemetery, founded in the 1660s as a burial ground for nonconformists, radicals and dissenters, holds the remains of John Bunyan, author of The Pilgrim's Progress, Daniel Defoe, who wrote Robinson Crusoe, and the poet and artist William Blake, among thousands of others.

In the 19th century, when it had already become a place of pilgrimage for nonconformists and radical reformers, the poet Robert Southey called it the Campo Santo (holy ground) of the dissenters. By the time it was finally declared full and closed in 1853, at least 120,000 people had been interred in the four acres.

"Paradoxically, the fact that many of those buried here would cheerfully have damned one another to hell on some minute point of theological dispute has brought them all together in this peaceful place," said David Garrard, the English Heritage historian who advised the government that such a unique place deserved the highest grade listing and protection. "Many of these people suffered a lifetime's persecution for their beliefs before coming to rest here."

He spent weeks poring over weathered inscriptions with an Edwardian guidebook - the most recent - in one hand and Dictionary of National Biography in the other.

Leftwing pilgrims of many shades visit regularly to lay wreaths, including Blake Society members, who come every August on the anniversary of the death of the man who wrote Songs of Innocence and the poem which became the hymn Jerusalem.

When the Independent newspaper was founded in 1986 with offices overlooking the burial ground, the editor Andreas Whittam Smith led a small delegation to lay flowers on the grave of Bunyan, who was in prison for his beliefs when he began writing Pilgrim's Progress, the book that is the most translated into other languages after the Bible.

Along with the Grade I listing for the burial ground as a landscape – putting it into the top 10% of the 106 listed cemeteries in England – 75 monuments are being individually Grade II listed including Blake's extremely plain headstone. His bones were lost in a partial clearance and landscaping in the 19th century and the original sites of more burials were lost as a result of 1960s landscaping to repair second world war bomb damage.

Bunyan's much grander tomb is to be listed Grade II*, along with Defoe's obelisk.

Bunhill Fields, just west of City Road in Islington, also provides the final resting place of Isaac Watts, the "father of English hymns", many still sung every Sunday; less famous hymn writer William Shrubsole, whose headstone is carved with three bars of his music; Susanna Wesley, mother of 19 children including Charles and John Wesley, whose London home, chapel and grave lie just across the road; and the engineer Thomas Newcomen, a pioneer of steam power.

Some monuments are being listed on their own merits rather than for their occupants; Henry Hunter, a Presbyterian minister and translator, earns a listing for his imposing 1801 obelisk, made of artificial Coade stone, while Eleanor Coade, inventor of the stone, is buried nearby. Dame Mary Page was buried in 1728 under a massive marble chest, with a long and excruciating inscription recording her last illness: "In 67 months she was tapp'd 60 times, had taken away 240 gallons of water, without ever repining at her case or ever fearing the operation."

Garrard says the burial ground is of unique importance as a vivid example of the London's old cramped cemeteries, with forests of headstones and thousands of graves jammed into every possible space, which shocked the Victorians and were almost all cleared as the large new garden cemeteries opened in the outskirts.

The land for the cemetery was originally leased from St Paul's Cathedral, which had used it as a dumping ground for bones being cleared from the charnel house and tiny burial ground around the church.

So many cartloads of bones were dumped that the land is said to have risen high enough to support a windmill. It was also designated as a plague pit, when – as chronicled by Defoe – thousands were dying in the city every week, but Garrard can find no evidence that plague victims were actually buried there.

It has been managed as a public open space by the City of London since the 1860s.