Forty people have died and more than 100 were wounded in an attack at a Moscow concert hall, Russia’s Federal Security Service said.
The attack, which Russian authorities are investigating as terrorism, is the deadliest in Russia in recent years.
Several gunmen burst into the concert hall on the edge of Moscow and sprayed visitors with automatic gunfire, killing and wounding dozens and starting a massive blaze.
Russian media reported that the venue’s roof was collapsing.
The attack comes days after President Vladimir Putin cemented his grip on the country in a highly orchestrated electoral landslide.
Russian news reports said that the assailants threw explosives, triggering the massive blaze.
Video posted on social media showed huge plumes of black smoke rising over the building, which can accommodate more than 6,000 people.
The attack took place as crowds gathered for a performance by Picnic, a famous Russian rock band.
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.
The prosecutor’s office said several men in combat fatigues entered the concert hall and fired at visitors.
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.
Russian authorities said security was tightened at Moscow’s airports and railway stations, while the Moscow mayor cancelled all mass gatherings scheduled for the weekend.
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 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.
Russia was shaken by a series of deadly terror attacks in the early 2000s during the fighting with separatists in the Russian province of Chechnya.
In October 2002, Chechen militants took about 800 people hostage at a Moscow theatre.
Two days later, Russian special forces stormed the building and 129 hostages and 41 Chechen fighters died, most of them from effects of narcotic gas Russian forces used to subdue the attackers.
And in September 2004, about 30 Chechen militants seized a school in Beslan in southern Russia, taking hundreds of hostages.
The siege ended in a bloodbath two days later and more than 330 people, about half of them children, were killed.
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