Nobel prize-winning Russian writer and dissident Alexander Solzhenitsyn, who shone a light on brutal Soviet Gulag camps, has died at the age of 89 and global tributes were made on Monday.
Recognizable in later life by his flowing beard and ascetic dress, he had been frail for several years and died of heart failure late on Sunday, his son Stepan said, quoted by ITAR-TASS news agency.
The Soviet Union's last leader, Mikhail Gorbachev, said Solzhenitsyn had helped undermine Stalinism by changing the views of millions through his writing.
Solzhenitsyn won the Nobel Prize for literature in 1970 after depicting in harrowing detail the Soviet labour camps, where he spent eight years from 1945.
He toiled obsessively to unearth the darkest secrets of Stalinist rule and his work ultimately dealt a crippling blow to the Soviet Union's authority.
He was eventually expelled in 1974 for his anti-Soviet views.
Russian President Dmitry Medvedev and Prime Minister Vladimir Putin sent the family their condolences and state television devoted large segments of its programming on Monday to the news of his passing.
His widow, Natalya, who is publishing his complete works, told Echo of Moscow radio that the writer lived "a difficult but happy life."
Gorbachev said Solzhenitsyn's name will go down in Russian history.
"Until the end of his days he fought for Russia not only to move away from its totalitarian past but also to have a worthy future, to become a truly free and democratic country. We owe him a lot," Gorbachev told Interfax news agency.
"Like millions of citizens, Solzhenitsyn lived through tough times. He was one of the first to talk about the inhumane Stalinist regime and about the people who experienced it but were not broken," Gorbachev said.
Solzhenitsyn played a key role in undermining Joseph Stalin's totalitarian regime, Gorbachev said. His works "changed the consciousness of millions of people."
French President Nicolas Sarkozy honoured Solzhenitsyn as "one of the greatest consciences of 20th century Russia."
"His intransigence, his ideals and his long, eventful life make of Solzhenitsyn a hero from a novel, an heir to Dostoyevsky. He belongs to the pantheon of world history. I pay homage to his memory," Sarkozy said.
Born in 1918 in Kislovodsk in the Caucasus amid the bloody aftermath of the Russian Revolution, Solzhenitsyn was initially a loyal communist.
But he was sentenced to eight years in the camps in 1945 for criticising Stalin in a letter to a friend and was to go on to survive cancer and a KGB assassination attempt.
He was released in February 1953, a few weeks before Stalin's death. He spent three more years in internal exile in the Soviet republic of Kazakhstan before moving back to Russia as a schoolteacher.
He burst onto the world of literature in 1962 with One Day in the Life of Ivan Denisovich.
Published with official approval during the thaw under Stalin's successor, Nikita Khrushchev, it described the world of the forced labour camps.
After its publication in the magazine Novy Mir, two subsequent editions totalling 850,000 copies sold out immediately.
Cancer Ward and The First Circle followed, both appearing in English in 1968.
Amid a crackdown under Khrushchev's successor Leonid Brezhnev, his writings were banned and Russians could read his texts only in clandestine editions.
He was awarded the Nobel Prize in 1970 but refused to travel to receive it for fear of not being allowed to return home.
By then Solzhenitsyn was sacrificing everything to his massive portrait of the camps, The Gulag Archipelago, covertly collecting information from 227 former prisoners.
The authorities discovered manuscripts of the book and in 1974 Solzhenitsyn was expelled by KGB chief Yury Andropov.
After a spell in Switzerland he moved to a remote village in Vermont, in the United States, where he devoted himself to his Red Wheel cycle, a fictionalised history of the run-up to the Revolution.
The world now discovered a Solzhenitsyn who was highly critical of Western ways and called for moral renewal based on Christian values.
His spectacular return to his homeland in 1994 proved something of an anti-climax. The new Russia was as alien to Solzhenitsyn as the United States had been, a finding he shared with audiences in gloomy televised harangues.
In June last year, Putin awarded Solzhenitsyn the State Prize, Russia's highest honour, praising his devotion to the "fatherland" in a lavish ceremony at the Kremlin.
Solzhenitsyn had an ambiguous relationship with the Kremlin, praising Putin for reviving Russia's greatness but also criticising the authorities for clamping down on democratic freedoms.