Russian law enforcement officers have detained 140 people at a Muslim prayer room in the capital, Moscow, as part of a search for extremist groups, Russian news agencies have quoted federal officials as saying.
A statement on Friday from the Federal Security Bureau (FSB), reported by Russian news agencies, said among those detained were 30 citizens of unspecified foreign countries.
Russian Information Agency said the detainees were taken to police stations for identification and questioning, and there was no immediate word on whether any would be charged.
State television showed men lining up and boarding a police bus after being detained.
No explanation provided
There was no indication of any link to the April 15 attack at the Boston Marathon, in which US authorities believe two ethnic Chechen brothers with roots in Russia's North Caucasus set off bombs that killed three people and wounded 264.
Authorities in Russia have so far declined to comment on the issue.
President Vladimir Putin said in a television appearance on Thursday that the Boston bombings justified his tough line against rebels in the North Caucasus and that Russia and the US must step up cooperation against them.
RIA reported that law enforcement authorities said some people who had been at the prayer room in the past had joined an extremist group in the North Caucasus.
But government critics say such raids are aimed largely at flushing out illegal migrants from the ex-Soviet republics of the South Caucasus and Central Asia, and to show that the authorities are taking action against extremists.
About 300 people were detained in a similar sweep in St Petersburg in February, and most were swiftly released.