Several gunmen burst into a big concert hall on the edge of Moscow and sprayed visitors with automatic gunfire, injuring an unspecified number of people and starting a massive blaze in an apparent terror attack.
It comes days after Russian president Vladimir Putin cemented his grip on the country in a highly orchestrated electoral landslide.
There were no immediate claims of responsibility for the raid, the worst terror attack in Russia in two decades that came as the fighting in Ukraine dragged into a third year.
Moscow mayor Sergei Sobyanin described the attack as a “huge tragedy”.
Russian news reports said that the assailants threw explosives, triggering a massive blaze at the Crocus City Hall on the western edge of Moscow.
Video posted on social media showed huge plumes of black smoke rising over the building.
White House national security adviser John Kirby said he could not yet speak about all the details but that “the images are just horrible. And just hard to watch”.
“Our thoughts are going to be with the victims of this terrible, terrible shooting attack,” Mr Kirby said.
“There are some moms and dads and brothers and sisters and sons and daughters who haven’t gotten the news yet. This is going to be a tough day.”
The attack took place as crowds gathered for a concert of Picnic, a famed Russian rock band, at the hall that can accommodate more than 6,000 people.
Russian news reports said that visitors were being evacuated, but some said that an unspecified number of people could have been trapped by the blaze.
Russia’s state RIA Novosti news agency reported that at least three people in combat fatigues fired weapons.
The state Tass news agency also reported the shooting.
Extended rounds of gunfire could be heard on multiple videos posted by Russian media and Telegram channels.
One showed two men with rifles moving through the mall.
Another one showed a man inside the auditorium, saying the assailants set it on fire, as gunshots rang out incessantly in the background.
More videos showed up to four attackers, armed with assault rifles and wearing caps, who were shooting screaming people at point-blank range.
Andrei Vorobyov, the governor of the Moscow region, said he was heading to the area and had set up a task force to deal with the damage.
He did not immediately offer any further details.
Russian media reports said that riot police units were being sent to the area as people were being evacuated.
The attack followed a statement issued earlier this month by the US embassy in Moscow that urged Americans to avoid crowded places in the Russian capital in view of an imminent attack, a warning that was repeated by several other Western embassies.
Mr Putin, who extended his grip on Russia for another six years in the March 15-17 presidential vote after a sweeping crackdown on dissent, earlier this week denounced the Western warnings as an attempt to intimidate Russians.
