At least 133 people were killed and more than 140 injured when gunmen attacked a packed concert venue on the outskirts of Moscow, Russia says
A large fire engulfed the roof of the complex and dramatic video shows panicked concertgoers taking cover as shots and explosions ring out
Russian President Vladimir Putin says all four gunmen have been arrested, and that the suspects were trying to flee to Ukraine - Kyiv says allegations of Ukrainian involvement are "absurd"
Putin calls the attack a "barbaric terrorist act" and announces a day of national mourning for 24 March
Muscovites are queuing to give blood for those injured and flower tributes have been placed at the scene of the attack
The US says it's credible that the Islamic State group could be behind the attack, after the group said it did it. Russia has not commented
Steve Rosenberg
BBC Russia editor
Billboards across Moscow are paying tribute to the victims
The "mourn" messages seen outside the venue have been replicated elsewhere in Moscow. All those I passed on my way home from work were showing the candle and the word "skorbim", along with the date of the attack.
The fire which has been raging at the Crocus City Hall has been "mostly eliminated", Moscow's governor has said. "There are still some pockets of fire," Andrey Vorobyov said on Telegram and emergency services were "able to enter the auditorium" - where many victims are believed to have been killed.
Image source, EPAFirefighters damp down at Crocus City Hall
More from the US now, as the White House confirms that it warned Russian authorities earlier in March about an attack possibly targeting "large gatherings" in Moscow.
"Earlier this month, the US government had information about a planned terrorist attack in Moscow - potentially targeting large gatherings, to include concerts," National Security Council spokeswoman Adrienne Watson said.
Washington "shared this information with Russian authorities", she added.
Gordon Corera, the BBC's security correspondent, earlier stated that the Kremlin shrugged these warnings off as "propaganda" - even after the US made its concerns public to Americans staying in Russia.
Will Vernon
BBC News, Washington DC
An Islamic State affiliate's claim that it was behind the Moscow attack would, if proved correct, be the latest in a number of Islamist-linked attacks in or against Russia in recent years.
A Russian plane was blown up over Egypt’s Sinai desert in 2015 with 224 people on board, most of them Russian citizens.
That attack was claimed by Islamic State, which later published a photograph of the explosive device.
In 2017, a bomb attack on the St Petersburg metro that killed 15 people was also linked to radical Islamists.
Militant groups have been fighting Russian forces in the North Caucasus region of the country for decades.
Large numbers of them flocked to Syria to join IS when the group was formed 10 years ago. Some militants that operate in the Russian North Caucasus have declared allegiance to IS, say experts.
But the specific group that has claimed today’s attack, ISIS-K, is the Islamic State affiliate in Khorasan - an old term for a region that includes parts of Iran, Turkmenistan and Afghanistan.
Counterterrorism experts say Russia has found itself “in the crosshairs” of ISIS-K, which frequently criticises President Putin in its propaganda.
It accuses the Kremlin of atrocities against Muslim people in military campaigns in Chechnya and Syria, as well as in the Soviet-era invasion of Afghanistan.
Image source, Getty ImagesFire crews work to extinguish the blaze
Image source, Getty ImagesA man wrapped in a heat blanket outside the concert hall
Image source, ReutersA digital billboard opposite the concert venue reads "We mourn" and the date of the attack
Image source, EPAPart of the concert hall building has been destroyed in the fire
It is now just passed 04:00 in Moscow (01:00 GMT) and President Vladimir Putin has yet to directly address the Russian people about yesterday's attack.
However, Russia's state-owned news agency Tass is reporting that Deputy Prime Minister Tatyana Golikova has relayed that the Russian leader has wished a speedy recovery to those wounded in the attack.
"The president wished everyone a speedy recovery and conveyed his gratitude to the doctors," Golikova was quoted as saying after meeting him.
In the last few moments, Russia's investigative committee has confirmed that at least 60 people have been killed in the attack.
"The bodies of the deceased are currently being examined. It is provisionally established that more than 60 people died in the terrorist attack. Unfortunately, the number of victims could rise," said the Investigative Committee charged with overseeing the response to the attack.
Moscow concert hall attack videos examined
BBC Verify's Merlyn Thomas has been examining dramatic videos taken by those in and around the hall.
Sarah Smith
North America editor
American officials have been quick to confirm that the US has intelligence confirming the Islamic State claim it carried out the attack in Moscow.
The BBC’s US news partner CBS has spoken to a source familiar with the intelligence, who says there has been a steady stream of information - dating back to November - about IS wanting to strike in Russia.
Some highly specific information was passed on to the Russian government, despite the frosty relations between Washington and Moscow.
It has not been confirmed, but it is likely, that this is information that was referred to in the state department warning two weeks ago: Americans in Moscow were told to avoid large gatherings, and specifically warned about concerts.
The State Department said the US was "monitoring reports that extremists have imminent plans to target large gatherings”. The warning did not at that time name the group it was referring to.
Gordon Corera
Security correspondent, BBC News
The statement from so-called Islamic State (IS) saying they were behind the attack came out on one of the group's official accounts.
That is not absolute confirmation - sometimes IS has issued false claims - however, a US official has told CBS, the BBC's US partner, that it has intelligence confirming the IS claim that it carried out the attack.
The nature of the assault – marauding gunmen killing as many civilians as possible – is typical of the way they operate. There are similarities, for instance, to the Bataclan attacks at a concert in Paris back in 2015 linked to the group.
The question of who was behind today's attack is highly charged because of the international context. Ukrainian officials - and their allies - are clearly worried Kyiv might be blamed by Moscow.
There is also the question of the US warning on 7 March about "extremists" attacking concerts. That public warning - based almost certainly on intelligence - appears to have been dismissed by Russian officials.
And so while there are always questions about who was behind an attack and if it could have been prevented, this time the pressure on Moscow may be more intense to see what they say and how they respond.
Shortly after news broke of the attack, pictures started to emerge online of several gunmen walking through the lobby of the Crocus City Hall, and shots can be heard.
Video shows gunmen in concert hall lobby
Russia's Health Minister Mikhail Murashko has said that 115 people are in hospital - five of them are children.
60 of the victims are in a serious condition, he added.
Steve Rosenberg
BBC Russia editor in Moscow
The latest information we have is that the attackers got away, possibly in a white car.
The search is on to find them, and to find out who they are.
Ukraine says it had nothing to do with this attack, and meanwhile Islamic State has claimed responsibility.
Russian authorities are yet to comment on that development. And Vladimir Putin has so far made no public comment, but we’re told he’s been kept up to date on events as they unfold.
Meanwhile security has been stepped up here in Moscow at key installations including airports and train stations.
Questions are being raised. Among them: How is it possible that armed men were able to get into a prominent concert venue, with thousands of people attending a concert, and carry out such a bloody attack – particularly since US authorities had recently warned about a similar scenario?
Foreign officials have been reacting to today’s events in Moscow:
Reuters news agency is reporting that Islamic State have claimed responsibility for the attack on its Telegram channel.
The report has not been independently verified. The group, also known as ISIS, has carried out attacks in a number of countries, however BBC security correspondent Gordon Corera notes that the group has at times in the past claimed responsibility for attacks that it had nothing to do with.
On 7 March, Russia's Federal Security Service (FSB) said it prevented an attack on a synagogue in Moscow that was being planned by an Islamic State cell.
The FSB said attackers opened fire during the attempted arrest and were "neutralized by return fire", but few other details were given about the alleged plot.
Image source, ReutersDmitry Medvedev, who briefly served as Russia's president, said in a statement on Telegram that those who are responsible for the attack should be "found and ruthlessly destroyed", particularly if they were inspired by "the Kyiv regime".
Now the deputy chairman of Russia's security council, Medvedev has used increasingly fiery rhetoric since Russia's invasion of Ukraine - especially when speaking about Ukraine and its Western partners.
We are getting more photos from outside the Crocus City Hall near Moscow, where at least 40 people were killed and more than 100 others were injured in a deadly attack.
Image source, Getty ImagesA view shows the top of the burning Crocus City Hall concert hall following the shooting incident in Krasnogorsk, outside Moscow.
Image source, Getty ImagesPeople outside the burning concert hall
Image source, Getty ImagesA police officer shouts as law enforcement responds to the attack
Image source, ReutersEmergency vehicles near the scene of the attack
Earlier, we brought you the news that all big events across Moscow had been cancelled this weekend.
Now, that has been expanded to the whole of Russia, by order of the culture ministry.
Image source, Getty ImagesHere's what we know so far about this evening's attack: