Russian police and security agents have detained 140 people at a mosque in Moscow on suspicion of involvement with Islamic extremism.
A statement from the Federal Security Agency reported by Russian news agencies said among those detained were 30 citizens of unspecified foreign countries.
The arrests come a week after the two suspects in the fatal Boston Marathon bombings were identified as Russian-born ethnic Chechens who sympathised with Islamic extremists.
The reports cited the agency as saying the mosque has previously been visited by people who had been involved in preparing or carrying out terrorist attacks.
A Chechen separatist insurgency that began in the 1990s increasingly took on a fundamentalist Muslim character and spread to neighbouring Russian Caucasus regions, including Dagestan, where Boston bombing suspects Tamerlan and Dzhokhar Tsarnaev and their family lived for a period before emigrating to the United States in 2002 or 2003.
The Tsarnaevs' parents later returned to Dagestan, and Tamerlan, who was killed in a shootout with police last week, made a long visit in 2012. Investigators are trying to find out details of what he did on the six-month sojourn, especially whether he met with any extremists.
Caucasus extremists have carried out gruesome attacks on civilians in Russia, including the 2004 seizure of a school in the town of Beslan that ended in the deaths of 330 people, about half of them children. They also claimed responsibility for the 2011 bombing of Russia's busiest airport, killing 36 people.
In 2011, US authorities questioned Tamerlan Tsarnaev at Russia's request, but found nothing that sparked their interest and stopped watching him.
On Friday, officials briefed on the investigation told the AP that US intelligence agencies had added the mother of the suspects, Zubeidat, to a government terrorism database 18 months before the bombings.
The mother called the information "lies and hypocrisy" and said she has never been linked to crimes or terrorism.