The cause of death has not been reported, but the patriarch was known to have suffered from heart disease.
On Thursday evening, Alexy II held a church service in one of Moscow's central cathedrals to mark a major religious holiday.
The church's ruling body, the Holy Synod, is due to gather for an urgent meeting in Moscow on Saturday following the patriarch's death.
Alexy II suffered a severe stroke in 2002. In April 2007, media reports said he was in a Swiss clinic in a serious condition or even dead. Church officials then confirmed the patriarch had undergone medical treatment in Switzerland, but denied the seriousness of his condition.
ViktorYanukovych sent a telegramme, expressing his condolenсes.
