Alexei Navalny was watching his favourite cartoon show, Rick and Morty, when he suddenly felt unwell. He was 21 minutes into an episode where Rick turns into a pickle. The late Russian opposition leader was on a flight back to Moscow after campaigning ahead of regional elections in the Siberian city of Tomsk in August 2020. Something was clearly wrong, and Navalny staggered to the bathroom.

There, he recalls, he had the grim realisation: “I’m done for.” He told a sceptical steward that he’d been poisoned and then lay down calmly in the aisle, facing a wall. Life didn’t flash before his eyes. Instead, he compares his experience of death – or near-death, as it turned out – to something from a dark fantasy. It was like being “kissed by a Dementor and a Nazgûl stands nearby”.

He is clear who gave the order to kill him with the nerve agent novichok: Vladimir Putin. Navalny calls Russia’s president a “bribe-taking old man” and a “vengeful runt” who sits on top of a “sinister regime”. The assassins were members of the FSB, the KGB’s successor agency. Navalny spent 18 days in a coma, waking up in hospital in Germany.

It was while recovering in Freiburg that he wrote the first part of his extraordinary memoir, Patriot. The second section consists of letters from prison, following his January 2021 return to Moscow, when he was dramatically arrested at the airport. Navalny says he embarked on an autobiography knowing the Kremlin could finish him off. “If they do finally whack me, this book will be my memorial,” he notes.

It took three years for his gallows humour prophecy to come true. Navalny died in February this year, his likely murder taking place in an Arctic penal colony. He was 47. Prison documents hint he was poisoned and the authorities removed the evidence: clothes, vomit, even snow he had come into contact with.

This is a brave and brilliant book, a luminous account of Navalny’s life and dark times. It is a challenge from beyond the grave to Russia’s murder-addicted rulers. You can hear his voice in the deft translation by Arch Tait and Stephen Dalziel: sharp, playful and lacking in self-pity. Nothing crushes him. Up until the end – his final “polar” entry is on 17 January 2024 – he radiates indomitable good humour.

Patriot includes a manifesto for how the country might be transformed: free elections, a constitutional assembly, decentralisation and a European orientation. Days before his murder, he predicted the Putin regime would crumble, while acknowledging the resilience of autocracies.

Trained as a lawyer, Navalny first attracted attention as a transparency activist. He bought shares in notoriously corrupt oil and gas companies and asked awkward questions at shareholder meetings. The Kremlin controlled TV and most newspapers, so Navalny wrote up his exposés online. In 2011 he founded FBK, an anti-corruption organisation which grew into a grassroots national movement run by volunteers. He expresses pride at the way his campaigns encouraged young Russians to take part in opposition politics. Police detained him for the first time in 2011 when he attended protests against rigged Duma elections. Undaunted, he stood two years later to be mayor of Moscow, coming second, before finding himself in an “endless cycle” of rallies, arrests and spells in custody.

The Kremlin’s response to all this was vicious. His brother Oleg was jailed after a fake trial, a provocateur threw green gunk at Navalny, blinding him in one eye. In 2016 he tried to run for president. His videos – of Putin’s tacky Sochi palace and former president Dmitry Medvedev’s dodgy schemes – attracted millions of views. Navalny writes movingly about his wife, Yulia, – whom he met on holiday in Turkey – as a soulmate throughout this period.

Given his understanding of Putin’s Stalinist methods, why did he return to Moscow? His answer is that the struggle to make Russia a normal state was “my life’s work”. He wasn’t prepared to dump his homeland or his convictions, he says. At first, jail conditions were bearable. Well-wishers sent sacks of letters and a tiramisu cake. In one dispatch, Navalny ponders the “amazing ability of human beings to adapt and derive pleasure from the most trivial things”, such as instant coffee.

Behind bars, he chatted to his cellmates and read. He preferred Maupassant to Flaubert and enjoyed Oliver Twist (though he wonders if Dickens got working-class dialogue right). The FSB spied on him 24/7; his warders wore body cameras and barked commands.

As conditions worsened, he made fewer diary entries. More criminal “convictions” piled up – for insulting a war veteran and for extremism. He was shuffled from one penitentiary to the next. Meanwhile, “perverted” prison staff refused to treat his back pain, prompting a hunger strike. He was categorised as a flight risk and woken throughout the night, put in a tiny punishment cell and denied his wife’s letters.

None of these privations stopped Navalny from denouncing Putin’s all-out invasion of Ukraine as an “unjust war of aggression”. The reason for the war is Putin’s desire to hold on to power at any cost, and an obsession with his “historical legacy”, he writes. Critics regard Navalny as a closet nationalist. But Patriot calls for Russia to withdraw its troops, respect Ukraine’s 1991 borders and pay compensation.

During one of Yulia’s visits, Navalny told her there was a “high probability” he would never get out of prison alive. “They will poison me,” he said. “I know,” she replied. He sketches out what this means – no chance to say goodbye, never meeting his grandchildren, “tasseled mortar boards tossed in the air in my absence”. Maybe an unmarked grave. His philosophy: hope for the best, expect the worst. His death is a terrible loss, for Russia and for all of us.

• Luke Harding’s Invasion: Russia’s Bloody War and Ukraine’s Fight for Survival is published by Guardian Faber (£10.99). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.

• Patriot by Alexei Navalny is published by Bodley Head (£25). To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.