A masked gunman killed 14 people at a midnight showing of the new Batman film in a suburb of the US city of Denver early on Friday, sparking pandemonium when he hurled a teargas cannister into the auditorium and opened fire on moviegoers.

Fifty others were wounded in the attack on the showing of <em>The Dark Knight Rises</em> in the Aurora suburb, some of whom were treated for the effects of tear gas, hospital officials said.

"This is a horrific event," Aurora Police Chief Dan Oates told a news conference, adding that a suspect was taken into custody in the parking lot behind the theatre.

Police said the Aurora gunman had appeared at the front of the theatre during the movie and released a canister which let out a hissing sound at the movie theater, located in a mall.

Dozens of police were at the scene, and the authorities evacuated the area while they checked for any explosive devices. They said there was no evidence of a second gunman.

Wendy Post, who was waiting to be reunited with her daughter at a local high school following the shooting, said her daughter told her the shooting erupted just minutes after the film began.

"She saw the exit door open and something was thrown across the screen, and then shooting started," she told local NBC affiliate 9News television. "It was chaos."

CNN quoted one witness as saying he saw a "guy slowly making his way up the stairs and firing, picking random people".

White House Press Secretary Jay Carney said President Barack Obama was notified of the shooting by his Homeland Security Adviser, John Brennan.

Colorado has suffered mass killings in the past. In 1999, two students opened fire at Columbine High School in the suburb of Littleton, near Denver, killing 12 students and a teacher.

Police spokesperson Frank Fania said the suspect was thought to be in his early 20s. He was carrying a knife, a rifle and a handgun when arrested, and one other gun was recovered from the theatre. The gunman was also wearing a bulletproof vest.

"He did not resist, he did not put up a fight," Fania said. The suspect's apartment building has been evacuated.

A witness told CNN: "We heard anywhere from 10 to 20 shots and little explosions going on. Shortly after that we heard people screaming. Then they came on PA system and said everyone needed to get out.

"As soon as we got out, there were people running around and screaming." He said friends had told him the gunman was wearing a gas mask.

Another witness said the gunman opened fire during a shoot-out scene in the movie, leading to confusion.

One man told the NBC affiliate that he was in the adjacent theatre watching another screening of the Batman movie when he heard gunshots and the theater filled with thick, choking smoke.

He saw bullets holes in the wall, and some people in his theatre were wounded: "I heard moaning%u2026they were in pain."

Fania said police received the first call about the shooting at 00:39 local time and responded within "a minute or two".

Local hospitals were alerted to a "mass casualty incident". Media reports said 10 people died at the scene and four died later in hospital.

A spokesperson for Denver's Swedish Medical Center said three people had been admitted to that hospital with gunshot wounds and were in critical condition.

Another hospital, Denver Health, had received six wounded, one of whom was in critical condition, spokeswoman Kalena Wilkinson said