A spokesman for the Moscow Patriarchy of the Russian Orthodox Church said Alexiy, who headed the powerful church for 18 years, died at his residence in Peredelkino outside Moscow.
The Church never commented on Alexiy’s health and did not immediately disclose a cause of death. But diplomats in Moscow had said he was suffering from cancer.
In a sign of his importance, Russian state television immediately ran a film showing highlights from Alexiy’s life, accompanied by the sound of tolling church bells.
“This is an irreplaceable loss for all Russian Orthodox people, wherever they live,” said Sergei Mironov, speaker of the upper house of parliament.
Patriarch of Moscow and All Russia since 1990, the Estonian-born Alexiy was a powerful and influential figure with close links to the Kremlin.
He oversaw a major religious revival in Russia after the collapse of the Soviet Union, with hundreds of new churches built across the country, monasteries reopened and seminaries filling with new priests.
Russia’s Orthodox Church is by far the biggest of the churches in the Eastern Orthodox communion and is the majority religion in Russia.
During his 2000-2008 presidency, Vladimir Putin, a former KGB spy under communism, was often seen with Alexiy attending major religious ceremonies and President Dmitry Medvedev has continued the tradition.
Former Soviet leader Mikhail Gorbachev said he was “so shocked that it is very hard for me to find words on the spot”, Interfax news agency reported. “I respected him deeply”.
Medvedev, who was on an official visit to India, was expected to make a statement shortly.
An unapolegetic conservative, Alexiy was outspoken in his defence of traditional Russian values and was critical of the Catholic church for what he said was its efforts to win converts among Russian Orthodox believers.
He stood in the way of a visit to Russia by the Polish-born leader of the Catholic Church, Pope John Paul II.
Both churches said they were open in principle to an historic meeting between the Russian Patriarch and Pope Benedict, who was elected in 2005. But despite several rounds of dialogue between bishops from both churches, Alexiy felt the obstacles were too great.
“Problems remain on the agenda for our bilateral relations with the Roman Catholic Church which demand real solutions,” Alexiy said in a speech in June to the Council of Bishops.
“Among them is the question of missionary activity of Catholics in traditionally Orthodox Russian lands”.
In a first reaction from the Catholic Church to Alexiy’s death, Bishop Brian Farrel, secretary of the Vatican’s Pontifical Council for Christian Unity, said:
“Patriarch Alexiy had to lead the Church in a period of great transformation. He knew how to carry out this task with a great sense of responsibility and love for Russian tradition.”