At least 60 people are dead and more than 100 are wounded after gunmen open fired at a rock concert at Crocus City Hall in a suburb of Moscow on Friday, Russian intelligence says.
A large fire broke out in the building, causing part of the roof near the theatre to collapse, but is now said to be mostly extinguished.
Islamic State group has claimed it carried out the attack, and said those responsible escaped.
The US says there is no reason to doubt this claim, with the White House saying it warned Russia of a potential attack on "large gatherings" in Moscow earlier this month
Russia has not commented.
Ukraine - currently at war with Russia - has denied any involvement.
Video footage obtained by the BBC shows the attackers firing indiscriminately at screaming civilians trying to run away while several explosions can be heard.
Security has tightened with events cancelled across Russia as the county's national guard searches for the attackers.
President Vladimir Putin has not addressed the nation directly, but according to a deputy has wished those injured a speedy recovery.
(BBC)
