Eyewitnesses said 6ft tall Holmes, dressed in black body armour and a gas mask, burst into the front of the theatre through a fire exit like the "Terminator", threw a smoke grenade and started firing at the audience with three different guns.
More than 60 people were shot – at least 12 of those fatally – as he walked up and down the aisle firing indiscriminately with a semi automatic rifle into the screaming crowd.
Reports were that a baby had been shot and that a six-year-old child was among the victims about half an hour into the midnight screening in Denver, Colorado.
Eyewitnesses said that they initially thought the attack was a publicity stunt for the movie, while other in nearby theatres said that they thought the gunfire was coming from the film.
After the attack the gunman was arrested at his car, which was parked in a car park at the rear of the cinema which police say was not a car park normally used by members of the public.
Later the FBI named the arrested man as James, Holmes, 24. He is said to live in an apartment in north Aurora, Colorado. He told police that his apartment was stacked with explosives and officers have evacuated the building and are searching his home.
A woman claiming to be Holmes’s mother told ABC News she had woken up unaware of the shooting and had not yet been contacted by authorities.
She immediately expressed concern that her son may have been involved. "You have the right person," she said. "I need to call the police... I need to fly out to Colorado."
The attack is the worst massacre in the US since 33 people were killed in 2007 at Virginia Tech. And it comes just 13 years after 13 people were killed at Columbine High School in Colorado, less than 30 miles from the cinema.
Afterwards Barack Obama urged the US to come together “as one American family” and pledged all necessary support for the people of Aurora, Colorado.
The shootings happened at 12.39am at a midnight showing of the film, which had attracted hundreds of Batman fans.
Evan, an eyewitness who was at the theatre with his wife and three children, said that initially people thought it was part of the show but it soon became "very real".
"The gunman came through the exit and threw a canister of smoke and then started shooting up the place with a semi-automatic weapon," he said.
"We thought it was part of the movie but it became very real, very quickly.
"He must have shot 19 or 20 rounds. People were scrambling left and right. Somebody behind us. Maybe a straggler was hit. He said 'I am hit'.
"He looked like the Terminator. He didn't say anything. He was just shooting and shooting and shooting.
"There were people crawling down the stairs trying to escape the hail of gunfire.
"It was a very bad scene. Very bad."
Another eyewitness revealed that the police stopped people leaving the cinema, fearing that there may be another gunman in the theatre.
Quentin Caldwell said that officers entered the cinema with shotguns and ordered those attempting to flee to put their hands up.
He said that police were wary of letting people leave the cinema, fearful that a gunman may be among them: “We got to an area near the projection booth and there were police officers saying ‘put your hands up, we need to make sure you are not armed’.
“Then he started asking his colleague ‘can they go, can we let them go?’ and then a female police officer said ‘run’.”
Holmes gave himself up in the car park of the Century Aurora 16 Movie Theater in a mall in the suburb of Aurora.
Police say the man made no attempt to resist and told them he had explosives at his home nearby.
A local reporter Justin Joseph of KDVR said that, according to witnesses, the moment Batman appeared on screen a man wearing body armour and a gas mask, and wielding at least one long gun, stood up and faced the crowd.
He said: "As people ran this gunman opened fire hitting people. Police sources have told us there are at least 10 bodies inside the cinema, most of them children or teenagers, and one baby.
"A baby was shot at point blank range, the family were gathered around screaming."
Brenda Stuart, from 850 KOA Radio, said: "People inside tell us they thought it was part of the movie. They heard what they thought were firecrackers, loud bangs and all of a sudden they saw the bullets flying.
"Police officers are carting the injured to the hospital in their own cars, not waiting for the ambulances."
A makeshift hospital was set up at the mall to treat those wounded in the attack.
A local police spokesman Cassidee Carlson said: "The scene is still very active and we have little information for release at this time."
A makeshift hospital was set up at the mall to treat those wounded in the attack.
Police said the gunman is from Aurora but gave further no details. They were searching his home after he suggested there may be incendiary devices there.
President Barack Obama issued a statement saying that he and the first lady, Michelle Obama, were "shocked and saddened" by the tragedy and that their "thoughts and prayers" were with the victims.
