"Then after both had exploded, that's when he began to shoot. He didn't say a word, not a word, the entire time ...
"He's shooting little kids, he's shooting adults, he's shooting our friends ... It's sad, it's really sad."
Some witnesses said the man stood up at the front of the theatre, but others said he kicked in an emergency door.
He was wearing a gas mask, a riot helmet and a bulletproof vest.
He used three weapons, including an AK-type rifle and a handgun, at least one of which may have been earlier hidden in the theatre.
He fired 10 to 20 shots, killing 10 people. At least two others died on the way to or in hospital.
Bullets flew through the theatre's walls into neighbouring screenings.
The Batman film was showing simultaneously in three cinemas in the complex and police were at the cinema to help control crowds.
Witnesses said police shouted "get down" and dragged bodies and people outside as people fell in the rush to escape.
One man was escorted out by police with blood seeping from his shoulder.
Sky News broadcast cellphone images showing bloodied moviegoers staggering through the foyer amid scenes of panic.
Another witness saw a police officer carrying an injured girl with gunshot wounds to her back.
Denver reporter Justin Joseph of KDVR, said the man stood up with a gun as soon as Batman appeared on screen. Moviegoers apparently initially thought his actions were part of the film premiere.
"As people ran, this gunman opened fire, hitting people," Britain's Telegraph newspaper quoted Joseph as saying.
"Police sources have told us there are 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."
Police, ambulances and emergency crews raced to the cinemas after frantic calls started flooding the 911 switchboard.
Hundreds of witnesses were taken by bus to a nearby school for questioning.
Some filmgoers were shown wearing Batman outfits, suggesting a man in a mask and body armour would not have looked out of place.
The gunman ran out a back entrance of the cinema into a carpark towards his vehicle, and was captured by police.
Sky News showed live pictures of a bomb-disposal robot examining the boot of the suspect's car in the carpark, where explosives were reportedly found.
Police later searched his apartment but found no explosives there.
Initial reports said two gunmen were involved, but later police said they were focusing on only one attacker.
Brenda Stuart, from 850 KOA Radio, told Sky News: "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.
President Barack Obama said he and wife Michelle were shocked and saddened.
"We are committed to bringing whoever was responsible to justice and ensuring the safety of our people and caring for those who have been wounded."
Nine victims were in a critical condition at University Hospital last night.
Staff said the youngest patient there was a 3-month-old, who they said was not seriously hurt.
Fox News reported they were treating both gunshot and shrapnel wounds.
Shooters on the rampage
Some of the worst US mass shootings
April 1999 - two teenage schoolboys shot and killed 12 schoolmates and a teacher at Columbine High School in Littleton, Colorado, before killing themselves.
July 1999 - a stock exchange trader in Atlanta, Georgia, killed 12 people, including his wife and two children, before taking his own life.
October 2002 - a series of sniper-style shootings occurred in Washington DC, leaving 10 dead.
April 2007 - student Seung-Hui Cho shot and killed 32 people and wounded 15 others at Virginia Tech in Blacksburg, Virginia, before shooting himself, making it the deadliest mass shooting in the United States after 2000.
December 2008 - a man dressed in a Santa Claus suit opened fire at a family Christmas party in Covina, California, then set fire to the house and killed himself. Police later found nine people dead in the debris of the house.
March 2009 - a 28-year-old laid-off worker opened fire while driving a car through several towns in Alabama, killing 10 people.
April 2009 - a man shot dead 13 people at a civic centre in Binghamton, New York.
November 2009 - US army psychologist Major Nidal Hasan opened fire at a military base in Fort Hood, Texas, leaving 13 dead and 42 wounded.
January 2011 - a gunman opened fire at a public gathering outside a grocery in Tucson, Arizona, killing six people including a 9-year-old girl and wounding at least 12 others. Congresswoman Gabrielle Giffords was severely injured with a gunshot to the head.
Colorado scene of Columbine killings
Colorado was also the scene of another of America's worst shootings - the Columbine High School massacre.
On April 20, 1999, two senior students, Eric Harris, 18, and Dylan Klebold, 17, embarked on a shooting spree, killing 12 classmates and a teacher.
They marched through the Littleton school wearing their trademark black trench coats and carrying guns and pipe bombs.
Harris and Klebold injured 21 other students and three were injured while trying to escape. The two killers then committed suicide.
The teens were reported to have banded together because they had been targeted by other students who had no time for them. The Denver Post reported at the time that the pair shouted: "This is revenge."
The pair had a fascination for Nazism and their attack came on the 110th anniversary of Hitler's birth.
Harris and Klebold were also interested in goth and metal music and were fans of rocker Marilyn Manson - who became a flashpoint for a controversy over the influence of violent music, movies and video games on teenagers.
The massacre also provoked speculation over gun control laws, the availability of firearms, the pressure on social outcasts and gun violence involving youths.
Series controversial for violent content
The Dark Knight Rises is the third film in the latest Batman series, which has sparked controversy for its violent content.
The latest instalment, directed by Christopher Nolan, features a villain called Bane, a masked terrorist whose plans for the fictional Gotham City bring Batman out of exile. It is planned to be the last in the current Batman series. The movie has a PG-13 rating in the United States "for intense sequence of violence and action".
It debuted in New Zealand cinemas on Thursday, also with a screening that started just after midnight. The film is rated M in New Zealand, which means it is unrestricted but recommended for mature audiences aged 16 and over.
The previous Batman blockbuster, The Dark Knight, was one of the most successful films in movie history, grossing more than a US$1 billion worldwide.
The Nolan Batman films, which star Christian Bale in the title role, have brought a darker, more violent tone to the franchise. The films have been critically acclaimed.
The Dark Knight - which also had a PG-13 rating - drew complaints from some viewers that its scenes were too graphic for a teenage audience. Heath Ledger, who played the Joker, died in 2008 before the movie was released.
The first film, Batman Begins, was released in 2005.
