Alexy-II, the head of the Orthodox Church and Russian Patriarch who was instrumental in the post-Soviet spiritual revival of the country, died in Moscow on Friday.
The Moscow and All-Russia patriarch has died at the age of 79, an official church announcement said. A senior church official quoted by RIA Novsoti said heart failure was believed to be the cause of the death.
President Dmitry Medvedev, who was to leave for Italy on Saturday from New Delhi after his maiden India visit, has cut short his tour and is returning home.
He described Alexi-II as a 'great citizen' of Russia.
'The rise of the Russian Orthodox Church, the real establishment of the principles of freedom of conscience and faith are directly connected with him and his name,' Medvedev said in his televised message recorded in New Delhi.
Paying tributes to the Patriarch, Prime Minister Vladimir Putin said: 'He was a fair man; his death is an enormous loss, a tragic event.'
Pope Benedict XVI said he was 'profoundly saddened' by the death of the Moscow Patriarch. 'I recall his courageous battle for the defence of human and Gospel values, especially in the European Continent,' he added.
Alexi-II took over the leadership of the Russian Orthodox Church in 1990 at a time when it was split and the very foundations of the atheist Soviet Union were shaking.
He played a key role in maintaining social stability and harmony after the Soviet collapse in 1991.
Russia's spiritual revival after almost eight decades of Communist oppression of the church is inseparable with the name of Patriarch Alexi-II, who as a bishop in 1986 had urged Soviet leader Mikhail Gorbachev to restore religious freedom in the country.
In 2007, he signed a reunification act with Metropolitan Laurus, head of the Russian Orthodox Church Outside of Russia (ROCOR), after almost 90 years of split. The foreign branch broke away in 1921, after accusing fellow clergymen in Soviet Russia of collaboration with the country's Communist regime.
The patriarch was known to have suffered from health problems in recent years and had returned this week from Germany after treatment in a Munich clinic.
The church's ruling body, the Holy Synod, is due to gather for an urgent meeting in Moscow to pick an acting leader and make arrangements for funeral, Bishop Mark, deputy head of the patriarchy's foreign relations department said.
The Patriarch known in his youth as Alexei Rediger was born in 1929 into the family of a Russian Orthodox priest of German descent in the Estonian capital, Tallinn.
Alexy-II stances on foreign policy issues were often seen as matching the Kremlin line, including his criticism of NATO strikes against Yugoslavia and the war in Iraq.