Videos posted online and survivors' accounts are revealing the horrifying scene at a Russian concert hall when camouflage-clad gunmen opened fire and people scrambled for safety —  with some playing dead on the blood-slick floor, according to reports. 

Russian officials on Saturday said at least 115 people were killed and hundreds wounded in the massacre Friday at the Crocus City Hall on the outskirts of Moscow. The ISIS claimed attack was the deadliest in Russia in the last 20 years.

The mayhem began around 8 p.m. when concert-goers began filling into the venue for a concert by the Russian rock group Picnic.

"Some people in brown clothing, I don't know who they were - terrorists, military, whoever - broke into the auditorium and started shooting at people with assault rifles," photographer Dave Primov told the BBC.

Primov, who watched the scene unfold from an upstairs balcony, said some people tried to squeeze between the seats seeking cover but found little protection as the gunmen continued to fire on the rows. 

A woman who was at the concert recounted how the attackers "ran in and started to shoot at people."

"I dropped to the ground and pretended to be dead. The girl next to me had been killed," the woman said in a video posted online by a group that claims to be linked to Russia's security services, The Telegraph reported.

Another woman said the gunmen "started shooting us from behind."

"Those who were closer to the glass doors immediately fell down," she continued. "My husband had a gunshot wound in his shoulder and leg."

She said the attackers were communicating with each other in a language other than Russian and shot anyone in their sights.  "We lay down and prayed," she said. 

A survivor described how one of the attackers "came up and started shooting straight at people" as she and her friends tried to hide in a corner near the entrance. 

"They had black hair and were shouting to each other but not in Russian," she said. 

The Islamic State has claimed responsibility for the mass killing but Russian officials announced they detained 11 suspects and four of them were nabbed in western Russia near the Ukrainian border. Ukraine has denied any involvement.

The gunmen immediately opened fire on people as they entered the concert hall, the BBC reported.

One of the security guards at the entrance said he and his colleagues hid behind an advertising board as the assailants approached.

"Those attackers passed 30 feet away from us - they started shooting randomly at people on the ground floor," the guard said, the BBC reported.

Some of the shooters tossed incendiary devices into the crowd, igniting a fire in the concert hall that eventually caused the roof to collapse on Saturday morning as firefighters battled the flames. 

A man watching the attack from the balcony said, "They threw some petrol bombs, everything started burning."

A woman buying ice cream with her 11-year-old daughter near the entrance said they were startled by the loud noises and then somebody yelled out to take cover.

"We rushed to the children, lay down and started setting up barricades from tables and chairs, and several wounded people came running to us," she told the BBC.

Some inside the auditorium thought the sounds of gunfire were part of the act.

Sofiko Kvirikashvili said she heard "some kind of endless burst of firecrackers - I turned around in the hall once, then again. The third time, I realised everyone in the hall had started running away in all directions."

Another woman said she was near the stage when she saw one of the shooters firing into the seats. 

"We ran behind the curtain and one of the Crocus employees in uniform told us to run and we ran out into the car park without any winter clothes," she said.