A gunman has opened fire at a Planned Parenthood clinic in Colorado Springs, killing three people and wounding injuring several others in a standoff that dragged on for five hours before he surrendered.

Mayor John Suthers paid tribute to police for hauling the gunman into custody without further bloodshed at the building, where people scrambled into a safe room in a desperate bid to save themselves.

The suspect first engaged in a protracted gun battle with police but ultimately handed himself over to officers, bringing an end to the violence, which played out under a steady snowfall in Colorado's second-largest city.

A police officer was among the dead, he said, while nine others - among them five police - were injured. None of the injured were in a serious condition.

Key points: Two civilians, one police officer killed

Two civilians, one police officer killed Nine others injured

Nine others injured Gunman taken into custody after five-hour siege

"I was watching the officers' movement within the building on security cameras and communicating to the officers who were in the building and it was the most incredible work of officers trying to minimise the fatalities and get this guy in custody," Mr Suthers said.

"I was just incredibly impressed. While it's a terrible, terrible tragedy, the way the police handled this kept it from being an even worse tragedy."

Mr Suthers also expressed sympathy for the victims' loved ones.

"Obviously, we lost two civilian victims, we mourn the loss of a very brave police officer," he said.

The dead policeman was identified as Garrett Swasey, 44, a campus officer at the University of Colorado at Colorado Springs who had raced to the scene of the shooting.

Officials later identified the suspect as 57-year-old Robert Lewis Dear of South Carolina, local media including The Denver Post reported.

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Vicki Cowart, president and CEO of Planned Parenthood Rocky Mountains, said she did not believe the centre had been specifically targeted.

"We're very pleased that our own security systems were operating at top-notch," she told CNN.

That included rushing into a safe room.

"They were able to hunker down and some got out early throughout the afternoon, but those that didn't were in a safe spot," Ms Cowart said.

The Planned Parenthood centre provides abortions, screening for sexually transmitted diseases, pregnancy testing and other services. The association has become a lightning rod for criticism by social conservatives.

The national Planned Parenthood office praised "the brave law enforcement officers who put themselves in harm's way" in the incident.

"We are profoundly grateful for their heroism in helping to protect all women, men and young people as they access basic health care in this country," the statement read.

Local media said that extra police were sent to protect the three other Planned Parenthood clinics in the Denver area.

Two women are evacuated from the Planned Parenthood building. ( Reuters: Rick Wilking )

Police initially feared Dear might have brought explosives, and officers carefully combed the crime scene after the gunman surrendered.

A number of people were inside the clinic at the time of the shooting, on what had been a regular working day at Planned Parenthood, a day after Thanksgiving.

Terrified people fled the building and rushed into the freezing open, some crying and pleading for help.

A sheriff's department SWAT team with at least one armoured vehicle and several ambulances had been on standby, as the freezing winter evening drew in.

The immediate vicinity was placed on lockdown and people were told to stay indoors.

Police spokeswoman Lieutenant Catherine Buckley said the shooter was armed with some kind of "long weapon" with a shoulder stock such as a rifle and some witnesses reported hearing automatic fire.

Authorities said they did not know the suspect's identity but believed he acted alone.

"We did get officers inside the building. They were able to shout to the suspect and make communication with him and at that point they were able to get him to surrender and he was taken into custody," Lieutenant Buckley said.

Gunman 'was shooting out from the windows'

Police placed the area near the facility in lockdown while the shooter was at large. ( AF: Justin Edmonds )

Police swarmed the area around the building after an emergency call reporting shots fired on Friday morning (local time), and officers ultimately confronted the suspect inside the building, Lieutenant Buckley said.

Television footage aired by CNN showed a number of clinic staff and patients being escorted safely into police vehicles from the building, which lies on the northwest side of Colorado Springs, about 112 kilometres south of Denver.

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Quan Hoang, the owner of a nearby nail salon, told CNN when he heard the gunfire break out, he feared a robbery was under way at a bank in the shopping area, which would have been bustling with people looking to pick up a bargain in the Black Friday sales.

"And we see cops, SWAT, the bomb squad, a whole bunch of people just trying to take cover around the Planned Parenthood area," he said by telephone.

"An officer came back in and said, 'Is everyone safe?' We asked him questions and he said they've barricaded him inside the Planned Parenthood and he was shooting out from the windows."

The FBI and agents from the US Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting local law enforcement investigators.

US president Barack Obama was notified of the shooting by his Homeland Security adviser, Lisa Monaco, and "will be updated on the situation as necessary", a White House official said.

Abortion a deeply divisive issue

As in much of the rest of the US, abortion is a divisive issue in Colorado, figuring prominently in attack ads during last year's US Senate race between incumbent Democrat Mark Udall and Republican challenger Cory Gardner, the winner of the election.

At least eight abortion clinic workers have been killed since 1977, according to the National Abortion Federation — most recently in 2009, when abortion doctor George Tiller was shot to death at church in Wichita, Kansas.

Clinics have reported nearly 7,000 incidents of trespassing, vandalism, arson, death threats, and other forms of violence since then, according to the abortion-rights group.

Colorado Springs was the scene of a mass shooting on October 31 in which a gunman killed three people near downtown before dying in a shootout with police.

Hillary Clinton and many others have reacted to the tragedy on social media using the hash tag #StandwithPP:

AFP/Reuters