A controversial doctor who provided late-term abortions was fatally shot as he walked into his Kansas church, and authorities said they have arrested a suspect in the case.
Abortion doctor George Tiller was murdered just after 10:00 am (1500 GMT) in the lobby of Reformation Lutheran Church in Wichita, Kansas, police and city officials said.
President Barack Obama decried the killing in a statement released by the White House.
"I am shocked and outraged by the murder of Dr George Tiller as he attended church services this morning," said Obama.
"However profound our differences as Americans over difficult issues such as abortion, they cannot be resolved by heinous acts of violence."
Wichita officials said the suspect, who was not identified, was arrested some three hours after the killing. Local media identified the suspect as a 51-year-old man, who would be returned to Wichita to be charged.
"It's an unfortunate incident to happen on a Sunday morning," Wichita police captain Brent Allred told reporters. "These things should not occur at a any time."
Long a lightning rod in the bitter US culture war over abortion, Tiller, 67, over the years had been picketed, bombed and even shot.
His was one of only three clinics in the United States that perform late-term abortions, which are performed on fetuses that would be viable outside the mother's womb.
Late-term abortions are legal in Kansas if two independent physicians agree that the mother could suffer irreparable harm by giving birth.
The shooting occurred just two weeks after Obama sought "common ground" over the divisive abortion debate in a controversial speech at one of the top Catholic universities in the United States.
President Obama has attempted to defuse one of the most emotive issues in US public life by arguing that while abortion should remain legal, the government should do all it can to limit unwanted pregnancies.
But Obama has angered the anti-abortion movement by reversing predecessor George W. Bush's restrictions on federal funding for embryonic stem-cell research and for family-planning groups that carry out or facilitate abortions overseas.
Obama's choice for health secretary, former Kansas governor Kathleen Sebelius, has been widely condemned by the anti-abortion movement because of her ties to Tiller.
His family released a statement saying Tiller "dedicated his life to providing women with high-quality heath care despite frequent threats and violence," according to the local KWCH TV station.
"We ask that he be remembered as a good husband, father and grandfather and a dedicated servant on behalf of the rights of women everywhere."
Tiller had been demonized by abortion opponents who regularly protested outside his clinic.
In 1986, someone placed a bomb on the roof of the clinic, seriously damaging the building. In 1993, Tiller was shot in both arms outside the clinic. Tiller recovered, and his assailant received an 11-year prison term.
Some 2,000 protesters were also arrested outside the clinic during summer-long demonstrations in 1991.
He was acquitted in March on charges that he performed 19 illegal abortions in 2003 in a case which his lawyer described as a witch hunt.
Tiller testified during the trial that he spent years under the protection of federal agents after the FBI discovered an anti-abortion assassination list in 1994 that listed Tiller as the top target.
US Attorney General Eric Holder said federal authorities offered protection to "appropriate people and facilities" across the country after the shooting Sunday.
"The Department of Justice will work to bring the perpetrator of this crime to justice. As a precautionary measure, we will also take appropriate steps to help prevent any related acts of violence from occurring," said Holder, the top US prosecutor, in a statement.
He called Tiller's murder "an abhorrent act of violence."
Holder added that federal law enforcement agents are coordinating with officials in Kansas to investigate the slaying.