UPDATE II: A Reddit user claiming to be one of the 50 wounded has posted pictures of his wounds on the social news site.
UPDATE: Aurora police have confirmed that the suspect, a white male in his 20s, is in custody.
A masked gunman killed 14 people at a midnight showing of the new Batman movie in a suburb of Denver early on Friday, sparking pandemonium when he hurled a teargas canister into the auditorium and opened fire on moviegoers.
Fifty others including children were wounded in the attack on the showing of "The Dark Knight Rises" in a mall in the Aurora suburb, some of whom were treated for the effects of tear gas, hospital officials said.
"This is a horrific event," Aurora Police Chief Dan Oates told a news conference, adding that a suspect was taken into custody in the parking lot behind the theater.
Police said the Aurora gunman had appeared at the front of the theater during the movie and released a canister which let out a hissing sound at the theater before gunfire erupted.
Dozens of police were at the scene, and the authorities evacuated the area while they checked for any explosive devices. They said there was no evidence of a second gunman.
Wendy Post, who was waiting to be reunited with her daughter at a local high school following the shooting, said her daughter told her the shooting erupted just minutes after the film began.
"She saw the exit door open and something was thrown across the screen, and then shooting started," she told local NBC affiliate 9News television. "It was chaos."
President Barack Obama, who was notified of the shooting on Friday morning by his homeland security adviser, John Brennan, urged Americans to "stand together" with the people of Aurora in the hours and days to come.
"Michelle and I are shocked and saddened by the horrific and tragic shooting in Colorado. Federal and local law enforcement are still responding, and my administration will do everything that we can to support the people of Aurora in this extraordinarily difficult time," Obama said in a statement.
"As we do when confronted by moments of darkness and challenge, we must now come together as one American family," he added.
Colorado has suffered mass killings in the past. In 1999, two students opened fire at Columbine High School in the suburb of Littleton, near Denver, killing 12 students and a teacher.
Police spokesman Frank Fania said the suspect was thought to be in his early 20s. He was carrying a knife, a rifle and a handgun when arrested, and one other gun was recovered from the theater. The gunman was also wearing a bulletproof vest.
"He did not resist, he did not put up a fight," Fania said.
Police said the suspect's apartment building had been evacuated and police were looking for explosives there after the suspect made statements about explosives in his residence.
CNN quoted one witness as saying he saw a "guy slowly making his way up the stairs and firing, picking random people." Another witness said the gunman opened fire during a shoot-out scene in the movie, leading to confusion.
"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," one witness told CNN.
"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.
One man told the NBC affiliate that he was in the adjacent theater watching another screening of the Batman movie when he heard gunshots and the theatre filled with thick, choking smoke.
He saw bullets holes in the wall, and some people in his theater were wounded: "I heard moaning...they were in pain."
Fania said police received the first call about the shooting at 12:39 a.m. local time (06:39 GMT) 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 Center 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.