A gunman wearing a gas mask opened fire in a crowded US movie theater screening the latest Batman film early Friday, killing at least 12 and wounding nearly 40, police and reports said.

Witnesses described a scene of chaos at the midnight debut showing of "The Dark Knight Rises," saying the gunman had set off tear gas bombs and opened fire in the packed theater in Aurora, a suburb of Denver, Colorado, AFP reports.

The shooter, identified by US media as 24-year-old James Holmes, was arrested at the rear of the theater, police said.

"Police were told the suspect 'appeared' at the front of one of the theaters, threw some type of gas or explosive device and started shooting," said an Aurora Police Department statement.

"When the suspect was arrested, police recovered one rifle, one handgun and a gas mask. A third gun is still inside the theater. The suspect ... offered no resistance when he was arrested," it added.

Ten people were killed at the scene, police said, announcing the overall death toll to at least 12. Earlier they had said at least 14 died. A local children's hospital reported six victims, the youngest aged six.

"The injured and dead include a wide age range. The conditions of the injured also vary, with some suffering very serious wounds," said the police statement.

Aurora police chief Dan Oates added that police had arrested the alleged gunman and that there was "no evidence" of a second, after earlier reports of two shooters.

President Barack Obama, who was in Florida, expressed shock at the shooting, after being told about it shortly before 5:30 am (0930 GMT).

"Michelle and I are shocked and saddened by the horrific and tragic shooting in Colorado ... my administration will do everything that we can to support the people of Aurora in this extraordinarily difficult time."

"We are committed to bringing whoever was responsible to justice. As we do when confronted by moments of darkness and challenge, we must now come together as one American family."

Chris Jones, who was in the theater, said the shooting began about 20 to 30 minutes into the screening.

"People just started dropping. We were on the floor trying not to get shot," Jones told the local KMGH-TV television station.

"I thought it was just fireworks. There was smoke, then I heard 'bam, bam, bam.' The gunman didn't have to stop to reload. Shots just kept coming, kept coming, kept coming."

Shots fired in one auditorium went through the wall and hit people in the auditorium next door. Jones said by the time he could get out, police were in the building.

Police chief Oates said the suspect had claimed to have explosives at his residence and that the apartment complex where he lived had been evacuated and was being searched.

Police spokesman Frank Fania told CNN he was wearing body armor and armed with a rifle and two handguns, adding that he had set off some kind of smoke device in order to sow panic.

Witnesses described chaos chillingly similar to that depicted in the Batman films -- in which maniacal villains terrorize Gotham City -- suggesting the movie could have inspired the shooting spree.

They said several audience members had shown up in costumes, which could have allowed the gunman to blend in with the melee and complicated the arrest.

Police did not provide details about the killed and wounded, but the PG-13 film would have attracted scores of teenagers.

One witness cited by the Denver Post said he was watching the film when he heard a series of explosions. Benjamin Fernandez, 30, said people ran from the theater and that there were gunshots as officers shouted "Get down!"

Aurora is barely 20 miles (32 kilometers) from the scene of the 1999 Columbine High School massacre, in which two students shot dead 13 people and wounded 24 before committing suicide.