Authorities say they hadn't determined a motive or whether the shooter had any connection to Planned Parenthood, a national women's health care provider that offers abortions at some clinics.
"We don't have any information on this individual's mentality, or his ideas or ideology," police lieutenant Catherine Buckley said.
Planned Parenthood said it did not know the full circumstances or motives behind the attack, or whether the organisation was the target.
A number of people were evacuated during the standoff -- some wrapped in blankets in the blowing snow -- to a nearby Veterans Administration clinic.
For several hours, the firing of a gun was the only indication police had that the shooter was in the building, Buckley said.
Officers finally made voice contact by shouting to him and convinced him to surrender, she said.
Video from the Denver Post showed a tall man in a white T-shirt being led away by police as snow fell on the frigid evening.
With the immediate threat over, authorities turned their attention to inspecting unspecified items the gunman left outside the building and carried inside in bags. There had earlier been concerns that he might have brought explosives.
Just before noon local time (8am NZT) today, three police officers were shot while responding to the initial report of shots fired. More than two hours later, the gunman shot another officer in an exchange with police inside the clinic, Buckley said.
The suspect surrendered about five hours after entering the building.
The shots sent people inside the clinic racing for cover. Jennifer Motolinia hid behind a table and called her brother, Joan, who said he heard multiple gunshots in the background.
"She was telling me to take care of her babies because she could get killed," he said of the mother of three.
He rushed to the clinic but was frustrated because a police barricade kept him from getting close.
"People were shooting, for sure ... There was a lot of gunfire. She [his sister] was calm, she was trying to hide from those people," Joan Motolinia said.
Police inside the building ushered staff and patients to the second floor without saying why, employee Cynthia Garcia told her mother-in-law, Tina Garcia.
Then Cynthia Garcia heard gunshots, but she couldn't tell where they were coming from, Tina Garcia told the Associated Press.
Police cordoned off the clinic, nearby medical offices and a shopping centre. Authorities ordered everyone in the area to take shelter where they were.
Denise Speller, manager of a nearby hair salon, said she heard as many as 20 gunshots in less than five minutes.
She told the Colorado Springs Gazette newspaper that she saw two officers near a bank branch, not far from the Planned Parenthood facility. One of the officers appeared to fall to the ground and the other knelt down to help and then tried to get the first to safety behind a patrol car, she said. Another officer told Speller to seek shelter inside the building.
"We're still pretty freaked out," Speller said. "We can't stop shaking."
Shelley Satulla said she saw five or six people put on stretchers and placed in ambulances lined up next to King Soopers shopping centre near the clinic.
Brigitte Wolfe, who works in the area of the shooting, told CNN that she saw about five officers behind a building with their guns drawn, as well as about three Swat vehicles and roughly seven police cars from her vantage point in a mall that's across the street from the clinic.
It was not immediately clear if the Planned Parenthood family planning centre had been specifically targeted. Abortion is one of the services Planned Parenthood provides for women, and the association has become a lightning rod for criticism by social conservatives.
The women's health care giant has also been mired in a months-long scandal after anti-abortion activists released secretly recorded videos showing the organisation's officials discussing use of aborted fetal tissue for medical research.
Planned Parenthood's Rocky Mountains president and CEO, Vicki Cowart, was defiant as today's drama played out. While emphasising that she didn't know for sure that the clinic was the gunman's target, she said: "We share the concerns of many Americans that extremists are creating a poisonous environment that feeds domestic terrorism in this country. We will never back away from providing care in a safe, supportive environment that millions of people rely on and trust."
During today's emergency, the White House confirmed that President Barack Obama had been briefed on the unfolding situation.
The New York Times reported that the shootout could be heard on the police scanner.
"We're exchanging gunfire," one officer said on the radio. "We are trying to keep him pinned down."
"Put gunfire through the walls," came a reply. "Whatever, we got to stop this guy."
CBS News reported that the gunman had fired bullets as he entered the clinic at 3480 Centennial Boulevard, which was guarded by security staff, and that law enforcement officials were concerned about the possibility of explosives being involved.
The Colorado Springs Gazette reported that one officer said on his radio that the suspect had shot out the back window of his police cruiser.
KKTV video from the crime scene captured teams of Swat officers armed with machine guns roaming the area outside the clinic with their weapons drawn. FBI agents and staff from the Bureau of Alcohol, Tobacco, Firearms and Explosives were also on the scene.
Planned Parenthood says on its website that it delivers reproductive health care and sex education to women and men throughout the United States. It runs nearly 700 health centres.
The group says that each year, 2.7 million people visit it for health services and information.
Colorado Springs is a city of 400,000 residents.
--AP, agencies