Denver's KOA radio station said a figure in a gas mask had opened fire at the showing in a mall in the suburb of Aurora and set off a smoke or tear gas bomb before shooting erupted.

"This is a horrific event," Aurora Police Chief Dan Oates told a news conference, adding that a suspect was in custody and that there was no evidence of a second gunman.

Dozens of police were at the scene, and the authorities evacuated the area while they checked if there was an explosive device in the area.

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

Police arrested a male suspect in the parking lot behind the theatre, police spokesman Frank Fania said. "He did not resist, he did not put up a fight," he said. The suspect's apartment building has been evacuated.

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

The gunman "appeared at the front of the screen" in the theatre and started shooting, Fania said.

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. The theatre also filled with thick smoke, he said.

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 spokeswoman for Denver's Swedish Medical Centre 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.