The four armed men walked calmly towards the metal detectors at Crocus City Hall, firing their automatic weapons point-blank in short bursts at terrified civilians who fell screaming in a hail off bullets.

Nearby, one witness named Natalya had just taken off her coat and was standing in line on Friday evening at the internal entrance to the 6,200-seat concert hall outside Moscow, where Soviet-era rock group "Picnic" was to perform its hit "Afraid of Nothing."

"The shots came from behind us," Natalya, who asked for her surname not to be used, told Reuters. She was just about to enter the stalls.

"It was loud, like a firecracker blast, fireworks, but like an automatic burst. I could hear it right behind me, not far away," Natalya said.

Then Natalya ran for her life.

"Everyone was screaming; everyone was running," she said. She ran to the nearby metro station through the cold Moscow night without her coat and escaped. "I experienced terrible emotions. It is simply a nightmare."

Reuters was able to piece together some of what took place at the concert hall from interviews with witnesses, video footage from the scene and Russian official accounts and media reports.

More than 143 people were killed and dozens more injured in the deadliest attack in Russia since the 2004 Beslan school siege. Islamic State claimed responsibility for the attack.

The Federal Security Service (FSB) said 11 people - including the four alleged attackers - had been detained in the Bryansk region, about 340 km (210 miles) southwest of Moscow, as they headed for the border over which they hoped to escape to Ukraine. Kyiv has denied any involvement in that attack.

Moscow regional Governor Andrei Vorobyov said 133 bodies had been recovered from the rubble in 24 hours and doctors were "fighting for the lives of 107 people". State TV editor Margarita Simonyan, without citing a source, had earlier given a toll of 143.

In a televised address, Putin said 11 people had been detained, including the four gunmen. "They tried to hide and moved towards Ukraine, where, according to preliminary data, a window was prepared for them on the Ukrainian side to cross the state border," he said.

Camouflage and combat vests

The men, clad in camouflage and with combat vests containing dozens of spare magazines, arrived at Crocus City Hall at about 7:40 p.m. in a minivan, leaping out of the back door and heading towards the entrance with their weapons, witnesses said.

They fired directly through the glass doors of the main entrance, shooting at anyone who crossed their path. Dozens of bodies, some in pools of blood, lay motionless on the marble floors and at the main entrance.

Some people smashed through reinforced windows and locked exits with their hands as shots echoed around 14-year-old hall just 12 miles west of the Kremlin.

After shooting people at the entrance, the men made their way into the hall itself just as hundreds of people were taking their seats for the concert which had been sold out.

"Some thought it was a kind of special effect of some sort," one witness, Anastasia Rodionova, told Reuters. "Then I saw with my own eyes how people were dropping and the automatic gun fire began."

"Your instinct for self-preservation kicks in, your eyes widen, where can I run? Then someone shouted to us: get up - don't lie down or they will shoot us all right now."

Rodionova said some men were able to smash down a door to the street and escaped.

Loudspeakers began to blare out that the concert was being cancelled for "technical reasons" and that people were requested to the leave the hall.

Rush to escape

Verified video showed people rushing for the exits as repeated gunfire echoed above screams. The attackers walked through the concert hall aiming and then firing at civilians with controlled bursts.

"They started firing at us. I fell down onto the floor," one injured woman told BRIEF media from a Moscow hospital bed. She crawled to the exits. "A girl next to me was killed."

Some ran to escape the men. Others cowered behind maroon seats. One woman said she told her friend to lie down behind the seats as the gun fire got louder and louder.

Russian investigators said the men began to set fire to the building. Some witnesses said they poured some sort of liquid on seating and curtains in several places before igniting it.

Witnesses told of leaping over fire, some with their clothes melting, to escape the blaze, sending flames and a plume of black smoke billowing into the night sky.

"They were shooting. We were in the far corner," Andrei, one witness, told Reuters at the scene. "We went down the fire escape routes to the back."

The roof collapsed and hundreds of firefighters battled for hours to contain the blaze which gutted the entire hall. All that was left were the charred iron support beams and the steel frames of hundreds of seats.

The Baza Telegram channel, which is known for its close contacts with Russian special services, said 14 bodies were found on evacuation staircases and 28 bodies had been found in one of the toilets.

The bodies of whole families were found, dead mothers embracing their dead children.

Moscow attack: Day of mourning after 137 killed at Crocus City Hall concert

Russia is observing a day of mourning after at least 137 people were killed in an attack on Friday evening at a packed concert venue in Moscow.

Flags are flying at half mast, many events have been cancelled and TV channels have updated their schedules.

More than 140 people were also injured when gunmen entered the Crocus City Hall, firing indiscriminately before setting it on fire.

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

On Saturday Amaq, the IS media outlet, posted an image of the four masked men they claimed were involved in the assault. Russia has not commented on the IS claim.

The group later released highly graphic footage from the attack. The video, which has been verified by the BBC as genuine, shows one of the gunmen opening fire on several people. The BBC will not be broadcasting this video.

Russian President Vladimir Putin has said all four gunmen who carried out the attack have been arrested.

On Sunday, Russia's Investigative Committee showed the detainees being taken from a van into a building, handcuffed and blindfolded.

In his televised address on Saturday, 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 in 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.

Ukrainian President Volodymyr Zelensky accused Mr Putin of seeking to "blame" Ukraine for the attack. "This worthless Putin, instead of dealing with his citizens of Russia, addressing them, remained silent for a day - thinking how to bring this to Ukraine," he said in his nightly address.

BBC Verify has been able to match details of two of the alleged attackers who appear in the IS video and an IS-released still image to videos of the arrested suspects posted on pro-Kremlin Telegram channels.

The US National Security Council said it warned Russia of a potential IS attack on "large gatherings", including concerts, in Moscow earlier this month. The Kremlin at the time dismissed that as "propaganda" and an attempt by Washington to meddle in the recent Russian election.

On Saturday, the White House said it condemned the "heinous" attack and described IS as "a common terrorist enemy that must be defeated everywhere".

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) on Friday.

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 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.

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."