Benjamin Cazenoves was one of hundreds of music fans who had crowded into the popular Paris concert hall the Bataclan on Friday night for a sold-out show to see US rock band Eagles of Death Metal.

A short time later, he was holed up on the first floor of the venue, badly injured, and posting news in real time of the unimaginable terror that was unfolding around him.

"Carnage .... Dead bodies everywhere," the Paris resident wrote in a series of posts on his Facebook page, as gunmen unleashed an attack on the concert hall about 9.30pm, local time.

According to witnesses, a number of gunmen shouting "Allahu akbar" had burst into the concert hall about an hour into the band's set and opened fire on the crowd, shooting indiscriminately.

The gunmen were mowing down the concert goers one by one, wrote Cazenoves. He believed they were heading towards him, and pleaded for someone to help.

"They are cutting down everyone. One by one. 1st floor soon!!!!" he wrote.

About 1000 people were attending the concert in the relatively small venue when the attack occurred, one of seven apparently coordinated attacks around Paris.

Julien Pearce, a radio reporter in France who also was at the concert, told CNN that he saw a number of young men dressed in black open fire on the crowd with assault rifles, firing at random as people screamed.

"It was a bloodbath," he said, adding that he saw at least 20 bodies.

"Some were dead. Others were very badly wounded," he said.

"It lasted for 10 minutes. Ten horrific minutes where everybody was on the floor covering their head."

Pearce saw two people he called terrorists enter the concert hall, "very calm, very determined", and firing "randomly".

He told CNN that the men wore black clothing but no masks. Pearce saw the face of one shooter, who he estimated was no more than 25 years old.

"He was like a random guy holding a Kalashnikov. That's all," he said.

Another concertgoer, Marc Coupris, told The Guardian that "it looked like a battlefield, there was blood everywhere, there were bodies everywhere".

"Everyone scrabbled to the ground," he said.

"I was on the ground with a man on top of me and another one beside me up against a wall. We just stayed still like that. At first we kept quiet. I don't know how long we stayed like that, it seemed like an eternity.

"I saw my final hour unfurl before me, I thought this was the end. I thought I'm finished, I'm finished. I was terrified. We must all have thought the same. Eventually, when a few gendarmes [police officers] came in slowly we began to look up and there was blood absolutely everywhere. The police told us to run."

A witness quoted by BFM television said he heard rounds of automatic rifle fire and people shouting "Allahu akbar!" at the Bataclan, the New York Times reported.

Terrified concertgoers climbed into the upper boxes at the 150-year-old music hall, or cowered under the seats, while the musicians ran from the stage. Some people were able to escape through the exits.

Other witnesses reported six to eight hostage-takers in the concert hall, looking for people hiding at the venue.

Police surrounded the building and, after a tense two-hour standoff, stormed the concert hall late on Friday night.

Michel Cadot, the Paris police prefect, said the attackers at the Bataclan blew themselves up with suicide belts as police closed in.

The Daily Telegraph in London reported that that three of four gunmen activated their explosives belts.

"Three of them blew themselves up and a fourth, who was also wearing a belt, was hit by police fire and blew up as he fell," one source said.

Police said at least 40 concertgoers were dead. Other reports said the death toll at the concert hall may be as high as 100.

The Bataclan's ground floor is shaped like a horseshoe, and can either have seating or standing room, depending on the act.

There is also an upper tier that runs around the curved edges of the horseshoe, which is where Cazenoves was believed to be hiding when he posted on Facebook. Friends later posted on Facebook that Cazenoves had escaped the theatre alive.

Eagles of Death Metal in a blues-influenced rock band with California roots, formed by friends Jesse Hughes, 43, and Josh Homme, 42. Homme also fronts another rock band, Queens of the Stone Age.

Despite its name, Eagles of Death Metal is not a death metal band. The name was chosen ironically, as the band members imagined a cross between death rockers and the American band The Eagles.

Mary Lou Dorio, the mother of Julian Dorio, the drummer for Eagles of Death Metal, told the Washington Post that her son and other band members managed to escape Le Bataclan when the attack began. Homme was not performing at the event. However, the fate of several crew members remained unknown, she said.

"It was awful," she said, adding that her son was at a local police station, where he was able to call his wife.

The shootings occurred near the former headquarters of Charlie Hebdo, the satirical newspaper that was attacked by terrorists in January.

At the time of those shootings, the Bataclan had supported the magazine by displaying the phrase "Je Suis Charlie", used in support of the attack's victims, at the front of the concert hall.