Police cordon off an scene at a movie theater where a shooting killed 14 people wounded 50 others during a showing of new Batman film "The Dark Knight Rises" in Denver in the early hours of Friday. [Photo/Xinhua]

DENVER, Colorado  - A masked gunman killed 12 people at a midnight showing of the new Batman movie in a suburb of Denver early on Friday, sparking pandemonium when he hurled a gas canister into the auditorium and opened fire on moviegoers.

About fifty others including children were wounded in the attack on the showing of "The Dark Knight Rises" in a mall 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 theater. He had initially put the death toll at 14, but a spokesman later told NBC the toll had been revised to 12.

Police said the Aurora gunman had appeared at the front of the theater during the movie and released a canister which let out a hissing sound before gunfire erupted.

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."

President Barack Obama, who was notified of the shooting on Friday morning by his homeland security adviser, John Brennan, urged Americans to "stand together" with the people of Aurora in the hours and days to come.

"Michelle and I are shocked and saddened by the horrific and tragic shooting in Colorado. Federal and local law enforcement are still responding, and my administration will do everything that we can to support the people of Aurora in this extraordinarily difficult time," Obama said in a statement.

"As we do when confronted by moments of darkness and challenge, we must now come together as one American family," he added.

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.