Gunman opens fire on Batman movie screening

At least 12 people have been killed after a 24-year-old gunman opened fire at a midnight screening of the new Batman movie in Denver Colorado.

Police say James Holmes entered the Century 16 cinema at the Aurora mall wearing a gas mask.

He fired teargas and shot at the movie-goers - including a baby.

One man was at the midnight screening of the new Batman movie The Dark Knight Rises when the shooting began:

"We proceeded to get out of there as quickly as possible ... My wife had fallen to the ground, partially trampled - I got her up and out of there and we took off."

"We got down on our hands and knees, the people in the higher ceiling, in the stadium seats - they were sitting ducks."

Holmes, a neuroscience student, was arrested at the scene.

Police say the shooter's apartment was booby-trapped and they've sent in a bomb disposal robot.

They've discovered jars of chemicals in Holmes' apartment.

PHOTOS: View images from the scene of the mass shooting in Colorado

Frank Landesville of the Aurora Medical Centre says some were burned by teargas.

He says they've seen patients with complaints of burning to their eyes, skin, mouth and nose, but they've been decontaminated and released.

The movie had been underway for 20 minutes before Holmes walked into the theatre and began his attack.

A spokesman says more than the initially reported 50 people have been shot:

"Our best available count on injuries as of right now is that 71 people were shot and that 12 are deceased. Two died at area hospitals, and 10 are deceased in the crime scene."

One witness says he was wearing protective clothing and a gas mask during the ordeal:

"He broke in through a door, and like all you could see is pretty much like the mask, just the gas mask, and he was probably one of the scariest images I've ever seen in my life and he just threw up the gun and started shooting, like at least like four or five rounds."

VIDEO: A witness recounts the shooting during the movie screening

Colorado's Governor John Hickenlooper says their hearts are broken, thinking of the friends and family of victims.

"This is the act apparently of a very deranged mind. This is a safe city and a safe state and a safe country."

Any motive is as yet unknown.

Meanwhile New Zealand's Prime Minister has broken from his party conference business to acknowledge the horrific shooting tragedy.

In his opening address to delegates John Key has described the movie theatre massacre as a tragedy and an unspeakable horror.

Photo: The cinema where Holmes opened fire (Getty Images)