Moscow, Russia - Low domestic birthrates and rising immigration from the former Soviet republics are producing explosive growth in Russia's Muslim community, which is on a track to account for more than half the population by midcentury.
"Russia is going through a religious transformation that will be of even greater consequence for the international community than the collapse of the Soviet Union," said Paul Goble, a specialist on Islam in Russia and research associate at the University of Tartu in Estonia.
Two decades ago, the Sobornaya Mosque was the only Islamic house of worship allowed in the Soviet Union. It stood largely empty, filling only with the occasional large foreign delegation from an Islamic country.
Today, it is one of four mosques in Moscow serving a Muslim population of about 2.5 million. On Fridays and holy days, it overflows with worshippers, leaving many to kneel on newspapers outside, their foreheads pressed against the concrete.
As in many countries with growing Islamic populations, tensions are also on the rise. Many ethnic Russians fear their country is losing its traditional identity, while many Muslims are offended by widespread discrimination and a lack of respect for their faith.
Russia's Muslim community is extremely diverse, including Volga Tatars, the myriad ethnicities of the North Caucasus and newly arrived immigrants from Central Asia. But they all share birthrates that are far higher than Russia's ethnic Slavs, most of whom are Orthodox Christians.
Russia's overall population is dropping at a rate of 700,000 people a year, largely because of the short life spans and low birthrates of ethnic Russians. According to the CIA World Factbook, the national fertility rate is 1.28 children per woman, far below what is needed to maintain the country's population of nearly 143 million. The rate in Moscow is even lower, at 1.1 children per woman.
Russia's Muslims, however, are bucking that trend. The fertility rate for Tatars living in Moscow is six children per woman, Mr. Goble said, while the Chechen and Ingush communities are averaging 10 children per woman. At the same time, hundreds of thousands of Muslims from Tajikistan, Uzbekistan and Kazakhstan have been flocking to Russia in search of work.
Russia's Muslim population has increased by 40 percent since 1989, to about 25 million. By 2015, Muslims will make up a majority of Russia's conscript army and by 2020 one-fifth of the population.
"If nothing changes, in 30 years, people of Muslim descent will definitely outnumber ethnic Russians," Mr. Goble said.
For many Slavic Russians, the prospect of becoming a minority in their country is terrifying.
"Russia is historically a Slavic, Orthodox Christian land, and we need to make sure it stays that way," said Alexander Belov, the head of the Movement Against Illegal Immigration, an increasingly powerful lobby that has organized dozens of rallies in recent months.
Attacks on mosques have been on the rise, and in September an imam in the southern city of Kislovodsk was fatally shot outside his home. During days of rioting in August, mobs chased Chechens and other migrants out of the northwestern town of Kondopoga.
Sensing the nationalist mood, Russian authorities have begun to crack down with laws designed to defend Orthodox Christianity and restrict the activities and movement of Muslims.
Most Muslims living in Russia are not immigrants, but natives of lands seized by the expanding Russian empire. Islam is recognized as one of Russia's official religions, along with Orthodox Christianity, Judaism and Buddhism.
But few nationalists distinguish between immigrants from ex-Soviet countries and non-Slavic Russian citizens. Mr. Belov, for example, said non-Slavs should be restricted from living in "traditional Russian lands."
"The Muslims of Russia have roots here. We have been part of Russia for centuries," countered Rusham Abbyasov, a spokesman for Russia's Council of Muftis, which represents Islamic spiritual leaders in the country. "It is not right to say that Russia is a Christian country. These people either don't know the history, or they are ignoring it."
Mr. Goble said that after decades of Soviet religious repression, most Muslims in Russia are secular. But with interest in Islam surging, they are open to being influenced by extremist ideas, "especially if they feel excluded from Russian society."
Western governments should encourage Russia to integrate Muslims into society and avoid discrimination, he said. "When Muslims are in the majority in Russia, they'll remember whether we spoke out for their rights or failed to."