Tributes to Soviet dissident author Alexander Solzhenitsyn poured in on Monday as the world mourned the death of one of Russia'sgreatest literary figures.

German Chancellor Angela Merkel praised the Nobel literature laureate as 'an outstanding writer and committed citizen,' in a letter of condolence sent to Russian President Dmitry Medvedev.

Solzhenitsyn, known as Russia's moral conscience for his unflinching accounts of the brutality of Joseph Stalin's repressions, died on Sunday in Moscow at the age of 89.

French President Nicolas Sarkozy described him as 'a figure in a novel, an heir to (19th century Russian author Fyodor) Dostoevsky, who belongs in the pantheon of world literature.'

'Through Solzhenitsyn's books, the world was able to learn with their own eyes the reality of the Soviet system, Sarkozy said in a statement issued in Paris.

Solzhenitsyn spent eight years of his life in the forced labour camps spread along the rail network from the Arctic Solovetsky islands to Kazakhstan. By his estimate the camps, known as gulags, processed over 60 million people between the 1920s and late 1940s.

He had worked tirelessly and unflinchingly to chronicle the horrors he and thousands of others suffered under Stalin's regime in his works, including the three-volume Gulag Archipelago.

US Secretary of State Condoleezza Rice called Solzhenitsyn a 'moral witness bringing the evils of the gulag to the attention of the world' through his writings.

'His brave and arduous lifes journey ... made him one of the 20th centurys most important voices in the struggle against the tyranny of totalitarian regimes,' she said in a statement.

The Swedish Academy, which awarded Solzhenitsyn the prestigious Nobel Prize in 1970, spoke of the 'historic contribution' he made with his writings about the Soviet prison system.

'He sparked an intellectual reckoning with Communism and Marxism that swept through the western world,' Swedish Academy permanent secretary Horace Engdahl told Swedish radio.

Fearing he would not be allowed to return to the Soviet Union, the author of One Day in the Life of Ivan Denisovich did not attend the Nobel award ceremony.

EU High Representative Javier Solana called Solzhenitsyn 'one of the greatest European writers of the 20th century' who 'will be remembered as an author who contributed to changing the course of history.'

Ukrainian President Viktor Yushchenko was one of the first world leaders to express his condolences, calling the Russian writer 'our own and dear to us all.'

'I wish to express heartfelt condolences over the death of the outstanding Russian writer and Nobel Prize winner,' said Yushshenko in a telegram sent to Medvedev.

Solzhenitsyn, his thin face covered by a full Orthodox beard in his last years, had been weak for several years and died of heart failure late Sunday, his widow Natalya said.

A day-long memorial vigil will be held for Solzhenitsyn on Tuesday at Russia's prestigious Academy of Sciences, which recognized the once-exiled author as a member in 1997.

'The death of Alexander Isayevich Solzhenitsyn is a heavy loss for the whole of Russia,' Prime Minister Vladimir Putin wrote in a telegram expressing his regret.

'His entire long, thorny life journey will remain for us a model of true devotion, selfless service to the people, motherland, the ideals of freedom, justice and humanism,' Putin said.