Moscow  - Soviet dissident writer Alexander Solzhenitsyn was widely mourned on Monday as Russia's moral conscience for his unflinching accounts of the brutality of Stalin's repression camps.

Solzhenitsyn, frail in the last year of his life, died of heart failure or a stroke late Sunday aged 89.

"He worked all day long as usual on Sunday and only at night felt unwell," his widow Natalya was quoted on Ekho Moskvye radio as saying.

"He hoped to die in summertime - he has died in summertime. He hoped to die at home - he has died at home ... Alexander Isayevich (Solzhenitsyn) has lived a difficult, but happy life," she said.

Former Soviet leader Mikhail Gorbachev said "it would be impossible to overestimate the value" of Solzhenitsyn's stubborn and fearless record of the horrors of the Soviet gulags.

His first expose, One Day in the Life of Ivan Denisovich, and the three volume Gulag Archipelago published in exile, "changed the consciousness of millions of people and forced them to rethink the past and the present," Gorbachev was quoted by news agency Interfax as saying on Monday.

"He was one of the few people to say to speak out strongly about the brutality of Stalin's regime and about people who were tested by it, but not broken," Gorbachev said.

Former president Vladimir Putin, whom Solzhenitsyn regarded as one of Russia's greatest reformers in latter years, sent his condolences to the author's son and widow on Monday.

That Solzhenitsyn vivid investigations into the Soviet regime's vast network of camps remains controversial was apparent in communist party leader Gennady Zuganov's comments on Monday.

Solzhenitsyn's "evaluation of the earliest Soviet epoch was nevertheless tendencious and one-sided," Zuganov said, adding "his personal tragedy, of course, weighed on his evaluation, but one can't transfer personal woes and tribulations on the life and exploits of a whole nation." (dpa)