AURORA, Colorado - A gunman wearing a full suit of tactical body armour, a helmet and a gas mask opened fire at a packed midnight showing of the new Batman film in a Denver suburb on Friday, killing 12 people after setting off two smoke bombs in the dark theatre.

Armed with an assault rifle, a shotgun and a pistol, the black-clad gunman wounded 58 others in the shooting rampage at a showing of The Dark Knight Rises at a mall in Aurora, turning the movie screening into a chaotic scene of dead or bleeding victims, horrified screams and pleas for help, witnesses said.

Police said 30 people remained hospitalised on Friday evening, 11 of them in critical condition.

Officers who arrived on scene within 90 seconds of the first emergency calls quickly took suspect James Eagan Holmes, 24, into custody in a parking lot behind the cinema, where he surrendered without a fight, Aurora police chief Dan Oates said.

Mr Holmes, a graduate student who authorities said had his hair dyed red and called himself "the Joker" in a reference to Batman's comic-book nemesis, was due to make an initial court appearance on Monday.

Police declined to say what, if anything, Mr Holmes said to them following his arrest. During an emotional press conference, Mr Oates would not comment on possible motives for the massacre that stunned the community and the nation.

Authorities were unable to enter Mr Holmes' apartment on Friday, saying he had booby-trapped it with what appeared to be sophisticated explosives. Police evacuated five nearby buildings and created a perimeter of several blocks and said they planned to detonate the suspected explosives with a robot on Saturday.

Meanwhile, police used dogs to search three buildings in a research complex at Anschutz Medical Campus at the University of Colorado at Denver where Mr Holmes had worked. A university spokeswoman said nothing suspicious was found.

Witnesses at the movie theatre told of a horrific scene, with dazed victims bleeding from bullet wounds, spitting up blood and crying for help. Among those taken to hospitals as a precaution was a baby boy just a few months old.

FUZZY PORTRAIT OF SUSPECT

Confusion reigned as shooting broke out during an action scene in the summer blockbuster. The suspect may have blended in with other moviegoers who wore costumes as heroes and villains, and some witnesses said they believed at first that his appearance was a theatrical enhancement to the film.

The shooting evoked memories of the 1999 massacre at Columbine High School in Littleton, 27km from Aurora, where two students opened fire and killed 12 students and a teacher.

It also resonated in the US presidential race. Both President Barack Obama and his Republican rival, Mitt Romney, toned down their campaigns, pulled their ads from Colorado and dedicated their scheduled events to the victims.

"My daughters go to the movies," Mr Obama told supporters at a campaign event in Fort Myers, Florida. "What if Malia and Sasha had been at the theatre as so many kids do each day?"

The gunman was armed with an AR-15 assault rifle, a 12-gauge shotgun and a Glock .40-calibre handgun, Mr Oates said. Police found an additional Glock .40-calibre handgun in his car, parked just outside the theatre's rear emergency exit, he said.

He was dressed entirely in black with a gas mask, ballistic helmet, tactical ballistic vest, throat guard, leggings and crotch guard, Mr Oates said, adding that Mr Holmes had purchased the weapons legally at three area gun stores in the past 60 days and bought 6000 rounds of ammunition.

A law enforcement official who asked to remain anonymous said the suspect had purchased a ticket, entered the theatre and propped open the emergency exit while he slipped out to "gear up" and return armed.

'SOCIALLY AWKWARD'

The portrait of Mr Holmes that emerged in the hours following the shooting remained fuzzy, with only a speeding ticket on his record and nothing to suggest he was capable of an outburst of gun violence.

He grew up in a middle-class San Diego neighbourhood and earned a degree in neuroscience from the University of California at Riverside before seeking his graduate degree from the University of Colorado.

People who knew him described him as kind toward children but said he had trouble finding work. He was also described as bright but was dropping out of his graduate programme at the time of the shooting, according to the university.

Billy Kromka, a pre-med student at the University of Colorado at Boulder who served as a research assistant alongside Holmes for several months last year, said he was astonished at the news.

"He basically was socially awkward but not to the degree that would warrant suspicion of mass murder or any atrocity of this magnitude," Mr Kromka, 19, said. "I did not see any behaviour he exhibited that indicated he would be capable of an atrocity of a magnitude like this."

Mr Holmes's family issued a statement of sympathy for the victims, saying "our heart goes out" to their loved ones and asking for privacy from the media.

In New York, police pledged to deploy officers at all 40 theaters where the film was playing, partly as a precaution against "copycats", and the Paris movie premiere was cancelled on Friday, event organisers said.

Director Christopher Nolan called the shooting an "unbearably savage" event for which he expressed "profound sorrow" to the victims and their families.

Reuters