The doors to the church have been opened, and the first people have been admitted, a RIA Novosti correspondent reported. The body of Alexy II, the Patriarch of Moscow and all Russia, will be brought to the cathedral in the evening, and will lie in state.
A service for Alexy II will be held, followed by an all-night vigil.
Alexy II, who led the Russian Church for 18 years, died at the age of 79 in his residency near the capital. He will be buried on Tuesday in the Annunciation side-chapel of Moscow's Epiphany Cathedral.
Alexy II became patriarch of the Russian Orthodox Church in 1990, shortly before the collapse of the atheist Soviet Union, and presided over a religious revival in Russia, with thousands of churches and monasteries being restored and hundreds of new ones built across the country.