Here's what is known about the shooting so far:
Unidentified gunmen opened fire at the Crocus City Hall in Krasnogorsk near Moscow on Friday evening during a concert of the band "Picnic".
Forty people are reported dead and more than 100 wounded after the shooting as of 1915GMT, Russian state news agencies said, citing Russia's Federal Security Service (FSB).
The Interfax news agency reported up to five gunmen were involved in the attack.
Russian prosecutors called the attack "an act of terrorism" and have opened a criminal case.
An eyewitness told Reuters reporters outside the venue that automatic weapons had been used.
Video posted online by Russian news agencies showed billowing clouds of smoke and flames rising from the venue building.
TASS reported that people remained inside the building, which is almost completely engulfed in flames and that others were trapped on the roof.
Special units of Russia's national guard as well as police and firefighters are on the scene, TASS said.
Moscow's mayor cancelled all large-scale public events over the weekend.
REUTERS
