A concert hall in Moscow Region was attacked by at least four people dressed in camouflage, Russian media reported.
Deaths and injuries have been reported after a "terrorist attack", as Russia's Foreign Ministry described it, at the Crocus City Hall. Specialist police are at the scene.
Video obtained by Reuters shows a large blaze and smoke rising from the hall.
Footage on social media showed gunmen inside the hall, while state media reported that some people were still inside.
Tass news agency reported that a third of the concert venue was on fire and that the roof was almost completely ablaze.
Russian Foreign Ministry spokeswoman Maria Zakharova called on the international community to condemn the incident, which it called "a monstrous crime".
Eyewitnesses have started to speak to Russian media about the attack. One man, called Vitaly, said he was on a balcony in the hall when he heard shots fired.
"We heard shots; we didn't know what happened at first. Then I saw some terrorists shooting people," he said.
"They threw some petrol bombs; everything started burning. We were led out towards an exit."
An anonymous eyewitness said parents were attending a ballroom dancing competition with their children when the shooting started.
"Children and teenagers were at the competition. People came and said there was a shooting. A crush ensued. The children were running out in their ballroom gowns."
US National Security Council spokesman John Kirby said the images of the shooting were "horrible and hard to watch".
He added there was "no indication" of Ukraine's role in the attack.
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