The concert hall was crammed with more than 1,000 people who had come to see a sold-out show by the American rock band Eagles of Death Metal.
MUST READ: U2 cancel Paris concert after attacks
Gunmen apparently burst into the crowd and started firing indiscriminately. At least two attackers were reported killed by security forces.
The shooting was one of a spate of seemingly coordinated attacks around the city.
The Bataclan, located in the 11th arrondissement in eastern Paris, is a famous venue near an area known for its nightlife.
It dates back to the 19th century, when it used to stage vaudeville "spectacles." It then went through a series of incarnations as a cinema and is now a multi-purpose concert hall.
The theater's colorful facade was done in the exotic "chinoiserie" popular at the time of its construction and once boasted an actual pagoda rooftop.
According to the Guardian, the Bataclan's horseshoe-shaped ground floor can fit seating or be made for standing room.
"It looked like a battlefield, there was blood everywhere, there were bodies everywhere," one survivor told the Guardian after being freed from the Bataclan.
"Everyone scrabbled to the ground. 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."
French President François Hollande called the carnage in the theater and elsewhere an assault "of unprecedented proportions." He then went to the Bataclan to survey the scene.
According to reports, Hollande declared there that France was "going to lead a war," presumably against Islamist militants. He said his country's response would be "ruthless."
MUST READ: California rock band's show cut short by attack on Paris venue
Relatives of members in Eagles of Death Metal who spoke to The Washington Post said the band managed to escape.
Mary Lou Dorio, the mother of Julian Dorio, the band's drummer, told The Post that her son was safe. She said the band members managed to flee the concert hall when the attack began but that the fate of several crew members remains unknown.
"It was awful," she said. Her son initially went to a local police station, where he was able to call his wife. He had left his phone on stage. Earlier in the day, Dorio posted a far happier photo from his visit to the French capital.
Dorio's wife, Emily, said she spoke to her husband only briefly.
"We are just holding our breath and saying prayers for everyone," she told The Post. "He called to say that he loved me and he was safe. Everyone on stage was able to get off."
