The incident occurred at a cinema showing the Batman film The Dark Knight Rises, a police official told NBC News.
Hayden Miller told Denver's 9News he was in an adjoining cinema when he heard "10-20 shots" and "small explosions".
"We heard people screaming," Mr Miller said.
he said a voice then came over the PA system and "told us to get out".
"Everyone was running, screaming," he said.
One suspect was in custody, police said. It was unclear whether other suspects were involved.
The injured were being transported to three local hospitals.
Witnesses reported hearing explosions.
KOA reports that 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.
The bomb squad is also on the scene.
Aurora, a suburb of Denver, is not far from Littleton, scene of the Columbine High School massacre in 1999 that claimed 13 lives.
