MOSCOW, August 4, 2008 (AFP) - Nobel prize-winning Russian writer and dissident Alexander Solzhenitsyn, who devoted his life to exposing the brutal Soviet Gulag, has died at the age of 89, bringing tributes from around the world 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 Sunday evening after going to bed at the end of a day's work, his son Stepan told AFP.

'He had been ill many years, but nevertheless he was still able to work every day and he was of completely sound mind all this time, so his death, in fact, was sudden,' he said by telephone.

The author was working on corrections to a 30-volume set of collected works the day of his death, Stepan said, adding that the family would 'treasure' the many condolences from people who knew Solzhenitsyn.

Solzhenitsyn's lying in state will take place on Tuesday ahead of his burial at the historic Donskoye cemetery in Moscow, Interfax news agency reported, citing a church official and a representative from the writer's foundation.

The Soviet Union's last leader, Mikhail Gorbachev, said Solzhenitsyn had helped undermine Stalinism by changing the views of millions through his writing and had fought until the end for the cause of democracy in Russia.

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 sent the family his condolences.

Solzhenitsyn's 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.

Solzhenitsyn played a key role in undermining Joseph Stalin's totalitarian regime, Gorbachev said. His works 'changed the consciousness of millions of people, forcing them to think more about the past and the present.'

In a telegram expressing his condolences to Solzhenitsyn's family, Prime Minister Vladimir Putin said the death was a 'heavy loss for Russia'.

'We will remember him as a strong, brave person with enormous dignity.'

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,' Sarkozy said.

Born in 1918 in Kislovodsk in the Caucasus in 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.

He was released in February 1953, a few weeks before Stalin's death and eventually became a maths teacher. He earned fame in 1962 with 'One Day in the Life of Ivan Denisovich'.

Published with official approval during the thaw under Stalin's successor, Nikita Khrushchev, its description of forced labour camps made a huge impact.

But Soviet leader Leonid Brezhnev banned his writings and Russians could only read clandestine editions of his work.

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 working on his massive labour camp portrait, 'The Gulag Archipelago.' He was expelled from the Soviet Union in 1974 after the authorities discovered manuscripts of the book.

After a spell in Switzerland he moved to a remote village in Vermont, the United States, where he devoted himself to his 'Red Wheel' cycle, a fictionalised history of the run-up to the Revolution.

During his exile, the world discovered a Solzhenitsyn who was 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. Solzhenitsyn praised Putin for reviving Russia's greatness but also criticised the authorities for clamping down on democratic freedoms.