The head of the Russian Orthodox Church, Patriarch Alexiy II, has died at the age of 79.
A spokesman for the Moscow Patriarchy said Alexiy, who led the powerful church for 18 years, died at his residence in Peredelkino, a former Soviet writers' colony outside Moscow.
The official cause of death was heart failure but diplomats said he had been suffering from cancer. The day before his death, Alexiy officiated at a religious ceremony in Moscow.
No date is set yet for the funeral.
Prime Minister Vladimir Putin called his death a "great loss".
Alexiy II was credited with helping restore the moral authority of the Russian Orthodox Church after decades of repression under communism.
Favoured by the KGB as he rose through the Church's ranks, he then oversaw its post-communist revival.
However, relations with the Roman Catholic Church remained frosty and he repeatedly refused to meet the late Pope John Paul II, or his successor, Benedict XVI.
The Russian Orthodox Church counts nearly 70% of Russia's population - about 100 million people - among its members.
The BBC reports Alexiy II was a hugely revered figure.
Former Soviet leader Mikhail Gorbachev said he was shocked by the death. "I respected him deeply," he said.
A spokesman for Russian President Dmitry Medvedev said he would return to Russia from India on Friday, cancelling a planned trip to Italy.
Born in Estonia
Born Alexei Rediger to a Russian Orthodox family living in Estonia in 1929, the future patriarch rose swiftly through the ranks of the Church after studying theology in St Petersburg.
By the age of 32 he was a bishop, by 35 an archbishop.
After moving to Moscow he served as the Patriarchate's chief administrator and the deputy head of the Church's external affairs department.
In the second post, he attracted criticism for what many thought was his unswerving obedience to the dictates of Soviet foreign policy.
Supporters argued that he was merely doing his best to win concessions for his persecuted church.
Alexiy II became the leader of the Russian Orthodox Church in 1990, shortly before the fall of the Soviet Union.
The BBC reports the church found itself with a prominent role rebuilding national self-esteem and morality in the post-Soviet period.
He insisted on his Church's right to be the sole national Church of Russia, bringing the scattered branches of the Russian Orthodox Church back under the control of the Moscow Patriarchate.
He also moved the Orthodox Church closer to political circles, often visiting the Kremlin and aligning himself with its foreign policy stances.
One reason for his differences with the Catholic Church was a dispute over land taken by the Greek Catholic Church from the Orthodox Church in Ukraine in the early 1990s.
He also accused the Catholic Church of missionary activity in traditionally Orthodox areas, and blocked Pope John Paul II's long-held intention of visiting Russia.
Relations improved after the 2005 election of Pope Benedict. The Pope said he was "profoundly saddened" by the patriarch's death.
Last year, Alexiy II presided over a union with a branch of the Russian Orthodox Church whose members fled abroad to escape the Bolshevik Revolution.
The ending of an 80-year schism with the Russian Orthodox Church Abroad was seen as a significant achievement for the patriarch.
Alexiy II also oversaw a major programme to restore and re-open hundreds of Russia's churches.