Russian President Vladimir Putin says all four gunmen who carried out a deadly attack at a packed concert venue in Moscow have been arrested.

At least 133 people were killed and more than 140 injured when the attackers stormed the building, firing indiscriminately and setting it alight.

Authorities said 11 people were detained in total, and the four gunmen were caught while heading to Ukraine.

The Islamic State (IS) group says it was behind the attack.

On Saturday, IS-linked news agency Amaq shared on Telegram an image of the four masked men they claimed were involved in the assault. Russia has not commented on the IS claim.

In a televised address, Mr Putin condemned the massacre - the deadliest in Russia for nearly 20 years - as a "barbaric terrorist act" and repeated earlier suggestions by Russian security services that the attackers had tried to escape to Ukraine.

Kyiv dismissed the claim that it was some way involved in the attack as "absurd".

"To suggest the suspects were heading to Ukraine, would suggest they were stupid or suicidal," Andriy Yusov, a spokesperson for the Ukrainian military intelligence directorate, told the BBC.

Reports of a serious attack inside the Crocus City Hall, in Moscow's Krasnogorsk suburb, began arriving at around 20:00 local time (17:00 GMT).

Up to 6,200 people were gathered there for a Friday night rock concert by veteran band Picnic when the shooting started.

One video posted online showed several men in camouflage striding across the concourse, where they opened fire at members of the public, before rearming and entering the hall.

Crowds of people were filmed screaming and running in panic as the men burst in. Others were seen taking cover behind their seats as the men fired into the auditorium.

Some of those who tried to escape from the gunmen were thought to have fled to the basement, and others to the roof.

"They were just walking and gunning down everyone methodically in silence. Sound was echoing and we could not understand what was where," concertgoer Anastasia Rodionova recalled.

Vitaly, another visitor, saw the attack unfolding from a balcony. "They threw some petrol bombs, everything started burning," he said.

Outside the hall, huge plumes of smoke filled the sky. A fire was later seen engulfing the hall's roof and facade. Tass state news agency reported that around a third of the building had been set alight.

Other reports spoke of explosions, the force of which shattered the glass on the top two floors of the structure.

A number of special forces units stormed the venue, while medical personnel, and dozens of ambulances were sent to the scene. Helicopters, circling overhead, tried to douse the fire.

The Russian Investigative Committee said that the attackers used flammable liquid to set fire to sections of the concert hall and that the deaths of the victims were caused by gunshot wounds and poisoning by combustion products.

Flowers were laid at a makeshift memorial outside the concert hall on Saturday
Reuters

On Saturday, a makeshift memorial was set up outside the concert hall where Muscovites lit candles and laid flowers. Others queued to donate blood for the victims of the massacre.

In Moscow and other towns and cities, electronic billboards displayed the image of a single burning candle and the word "Skorbim" - "we mourn."

President Putin has declared Sunday to be a day of national mourning and weekend events have been cancelled across Russia.

Memorials also appeared outside Russian embassies in several countries, including the UK.

Semyon Khraptsov, whose wife attended the concert last night and is now missing, said he had tried calling five hospitals but the phone lines were all busy.

"I am in complete panic. I don't know what to do, [I'm in] complete hopelessness," he said.