DENVER, Colorado - A masked gunman shot dead 10 people and wounded 30 to 40 others at a showing of new Batman movie "The Dark Knight Rises" at a movie theatre in the US city of Denver, local radio station KOA reported on Friday, citing witnesses.

The 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. It said initial police estimates were of 30-40 injured.

Dozens of police were at the scene, CNN footage showed.

KOA said authorities were evacuating the area while they checked if there was an exploding device there.

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

It said local hospitals were alerted to a "mass casualty incident".

Police did not immediately respond to requests for comment, but a spokeswoman for Denver's Swedish Medical Center said three people had been admitted to that hospital with gunshot wounds and were in extremely critical condition.

Another hospital, Denver Health, had received six wounded, one of whom was in critical condition, spokeswoman Kalena Wilkinson said.