Patriarch Alexy II, the head of the Russian Orthodox Church, has died at his residence outside Moscow, a church spokesman has announced.
There was no immediate word on the cause of death of the patriarch, who was 79, but he had been sick for some time.
Within minutes of the announcement of his death, state television broadcast video footage of him and former and current officials voiced their condolences.
"I am shocked," said Mikhail Gorbachev, the last Soviet president who held power in the Kremlin when Alexy II became the head of the church in 1990.
"It is hard to find words. I had immense respect for him."
Sergei Mironov, the speaker of the upper house of parliament, said: "This is an irretrievable loss for all Russian Orthodox people, wherever they live."
A spokesman for Dmitry Medvedev, the Russian president, said he would return to Moscow from India on Friday, calling off a planned trip to Italy.
Patriarch Alexy II was an establishment figure who restored the authority of the church after decades of Soviet repression.
Born Alexei Ridiger, Alexy II made his ecclesiastical career at a time when the church was controlled by Soviet authorities before forging an alliance with the new Russian state under presidents Boris Yeltsin and Vladimir Putin.
The patriarch had a benign expression and moral authority among millions of Russian believers but his personality was always locked in by the deeply hierarchical nature of his role.
Alexy II took stances on foreign policy issues that often reflected the Kremlin line, criticising Nato strikes against Yugoslavia, the US-led war in Iraq, and defending the rights of ethnic Russians in the former Soviet Union.
But his role in the international arena was marked above all by wariness of Catholics, whom he accused of "proselytism", and he refused repeatedly to meet Roman Catholic Pope John Paul II and his successor Benedict XVI.
The main reason for the row was a property dispute between the Catholic and Orthodox churches in Ukraine, where the Greek Catholic Church, which was banned by Stalin and dispossessed, took back hundreds of parishes from the Orthodox church at the beginning of the 1990s.
The creation of four Catholic dioceses in Russia also created suspicion among Orthodox leaders. Several rounds of negotiations between Catholic and Orthodox officials failed to smooth differences.
Alexy II was a unifying Orthodox figure, who helped engineer a union with a branch of the Russian Orthodox church that separated from Moscow-based church authorities after the 1917 Soviet revolution.
Ridiger was born on February 23, 1929, in then independent Estonia, the son of an Orthodox priest.
He worked in two cathedrals after Estonia became part of the Soviet Union and entered a religious seminary during Stalin's rule.
He married but then divorced in order to become a monk in 1961 during the anti-religion campaigns launched by Nikita Khrushchev, the Soviet leader .
He was soon promoted to become an Orthodox bishop.
Ridiger had a successful career under Leonid Brezhnev at a time when the Orthodox church was effectively controlled by the KGB and dissident priests were thrown into jail.
The future patriarch conformed and rose rapidly through church ranks, becoming number two in the influential external affairs section of the patriarchate.
Despite his ties with the Communist establishment, he made some efforts to curb Soviet repression, including keeping a famous convent in Estonia open despite the threat of closure.
He became patriarch in 1990, shortly before the fall of the Soviet Union.
In close collaboration with Boris Yeltsin, Alexy II used his relations with the authorities to rebuild the influence of the Orthodox church.
Seminaries were restored, churches were rebuilt and church finances were greatly boosted by income from customs duties granted by the Russian government during the 1990s.
The lavish Christ the Saviour cathedral in central Moscow, which was destroyed under Stalin and replaced by an open-air swimming pool, was rebuilt in full splendour during Alexy II's patriarchate.
Religion gained influence in schools, prisons, hospitals and the armed services.
Within the church, Alexy II was never an innovative leader and opposed himself to liberal policies, but he also rejected deeply anti-Semitic and nationalistic currents in religious thinking.
The patriarch died at a time when the Russian Orthodox Church had not yet determined its preferred status, as an institution closely allied with political authorities or a church more in tune with the Russian people.