MOSCOW (Reuters) - Russia marked its adoption of Christianity in 988 on Wednesday with a new public holiday, the latest show of Kremlin support for an Orthodox Church that has grown increasingly powerful since the fall of Communism.
Rights groups have criticized the new holiday, approved by President Dmitry Medvedev, as undermining Russia's secular constitution and members of the country's large Muslim minority have complained that it excludes them.
Marking the anniversary Patriarch Kirill, head of the Russian Orthodox Church, held a liturgy in Kiev, the capital of modern Ukraine and mediaeval Kievan Rus, whose leader Prince Vladimir made Christianity the state religion more than 1,000 years ago.
Kievan Rus is seen as the precursor of modern-day Russia, Ukraine and Belarus.
"Facing aggressive atheism and resurgent paganism we remain firm in our belief in God," Kirill, clad in a flowing gold cloak, told thousands of followers in Kiev's historic Pecherska Lavra monastery.
The Orthodox Church has undergone a revival since the fall of the Soviet Union almost 20 years ago ended decades of repression under Communism, and Russia's leaders have endorsed it as the country's main faith.
Prime Minister Vladimir Putin said on Monday that Russia's adoption of Christianity had brought it closer to Europe.
"This was an event of colossal significance ... Russia made a historical choice," he said after lighting a candle in Veliky Novgorod's Saint Sophia Cathedral, considered Russia's oldest.
Religious ceremonies were held across Russia on Wednesday. Scores of worshippers held gold crosses as baby boys and the elderly were baptized in a river in the southwestern city of Stavropol.
Hundreds of believers also gathered near Red Square in Moscow and waved Russian flags while listening to hymns.
"It is really cool that this is finally happening. It's good young people know about our history," said Anton, a 22-year-old artist. Retired engineer Zina looked on. "I regularly go to church and maybe this will encourage others," she told Reuters.
The trend toward consolidation of the church as a national force in Russia has worried its 20-million strong Muslim population -- a seventh of Russia's people -- as well as those who believe church and state should be strictly separated.
The Russian patriarch also presides over the biggest branch of the Ukrainian Orthodox Church, although some Ukrainian clergy have declared themselves independent of Moscow and Kirill's visit to Kiev has angered some Ukrainian nationalists.
July 28 was celebrated in Russia en masse twice before -- in 1888 and 1988 -- though Wednesday's holiday was the first time it was marked on a state level.
Its millennium celebration in 1988 under Soviet leader Mikhail Gorbachev was widely considered a turning point for the church's revival while Russia still lived under Communism.
Muslim lawmakers have asked for a national holiday to mark the arrival of Islam in Russia more than 1,000 years ago.
Additional reporting by Olzhas Auyezov in Kiev and Gleb Bryanski in Novgorod; editing by David Stamp