The liturgy is being led by Metropolitan Kirill of Smolensk and Kaliningrad, who was appointed interim leader of the Russian Orthodox Church on Saturday, the day after Alexy II died of heart failure at the age of 79.
White-robed bishops from all over Russia chanted prayers around Alexy's coffin, placed in the center of the cathedral and adorned with roses. Russian President Dmitry Medvedev and Prime Minister Vladimir Putin are attending the service.
After the four-hour service, which began at 9:00 a.m. (06:00 GMT), the patriarch's body will be taken across town to the Epiphany Cathedral, where he will be buried.
Thousands of people have gathered outside the Cathedral of Christ the Savior to pay their respects to the patriarch, credited with leading a revival of the Church after the fall of communism. The cathedral had been demolished under Joseph Stalin and was rebuilt during the 1990s.
People attempting to reach the cathedral by car have brought traffic to a standstill throughout much of the city center, prompting traffic police to issue a request that drivers avoid using their vehicles until late afternoon.
The funeral procession to the Epiphany Cathedral, located on Spartakovskaya Street, will begin at 1:00 p.m., and the burial is set for 2:00 p.m.
Alexy II became patriarch in 1990, a year before the Soviet Union's collapse, and presided over the rebuilding of thousands of churches and monasteries and the construction of hundreds of new churches across the country.
He was credited with restoring ties with the Russian Orthodox Church Outside of Russia, founded by immigrants who fled the Bolsheviks, but was criticized by some for close ties to the Kremlin. He has also been accused of working with the KGB during the Soviet era, allegations denied by the Church.
