WICHITA, Kan. (AP) — A jury took just 37 minutes Friday to convict an anti-abortion activist of murder for putting a gun to the head of abortion doctor George Tiller and pulling the trigger in the foyer of a church.

Attorneys for Scott Roeder had hoped to argue for a lesser conviction of voluntary manslaughter, based on the defendant’s belief that the killing was justified to save the lives of unborn children. But the judge threw out that defense a day earlier, leaving the jurors to choose between a murder conviction or acquittal.

Roeder, 51, of Kansas City, Mo., admitted his actions on the witness stand. He faces a mandatory sentence of life in prison.

Defense attorney Mark Rudy described his case as ‘‘helpless and hopeless.’’

‘‘I’ve never seen anyone lay himself out as much as Mr. Roeder did,’’ Rudy said after the verdict, referring to his client’s confessions.

Prosecutors carefully sidestepped the abortion debate as they laid out a case painting Roeder as a cold and careful killer who methodically planned his attack on Tiller, one of the only doctors in the nation to offer late-term abortions.

But both sides of the abortion debate lined up to respond to the verdict.

Abortion-rights advocates said the decision would send a message to the militant fringe of the anti-abortion movement.

Vicki Saporta, president of the National Abortion Federation, said she hoped the verdict would be a ‘‘deterrent to those that that are considering following in Roeder’s footsteps.’’

‘‘While the verdict won’t bring back Dr. Tiller, it was very important justice was done today for the safety and security of other abortion providers across the country and women’s ability to access abortion care,’’ Saporta said.

Troy Newman, president of Wichita-based Operation Rescue, said ‘‘pro-life was not on trial. An insane man doing an insane thing was on trial.’’

He said Roeder ‘‘could have saved us about three weeks of taxpayers’ time and the city’s agony if he would have just confessed to it up front.’’

Roeder could be considered for parole after 25 years. But prosecutor Nola Foulston said she would seek to ensure that he serve at least 50 years before being eligible. Sentencing was set for March 9.

In the courtroom, Tiller’s family held hands and fought tears as the verdicts were read. Tiller’s widow, Jeanne, later released a statement saying that the jury had ‘‘reached a just verdict.’’

The family said it wanted Tiller to be ‘‘remembered for his legacy of service to women, the help he provided for those who needed it and the love and happiness he provided us as a husband, father and grandfather.’’

In a November interview with The Associated Press, Roeder admitted shooting Tiller in the foyer of the Wichita church where the doctor was serving as an usher. On the witness stand, he testified he felt that Tiller placed the lives of unborn children in ‘‘immediate danger.’’

During closing arguments Friday, Rudy urged the jury to reject the murder charge. ‘‘No one,’’ he said, ‘‘should be convicted based on his convictions.’’

Rudy mentioned leaders who stood up for their beliefs, including Martin Luther King Jr. They were ‘‘celebrated individuals (who) stood up and made the world a better place.’’

‘‘They leave their marks based on their words and deeds,’’ Rudy said.

But prosecutor Kim Parker said Roeder was ‘‘simply guilty of the crime he has been charged with.’’

Prosecutor Ann Swegle told jurors to use their ‘‘common sense’’ and find Roeder guilty based not only on the state’s case but also on Roeder’s own testimony in which he described how he killed Tiller in a ‘‘planned assassination.’’

‘‘There could be no other verdict in this case,’’ she said.

Wearing a dark suit with a red tie, Roeder sat expressionless as the verdict was read. He moved his head toward the judge and to the jury as each juror confirmed the decision.

Tiller’s Wichita clinic was the focus of many protests and had been under investigation by a former state district attorney who accused the doctor of skirting Kansas’ abortion laws. In 2009, Tiller was acquitted of misdemeanor charges of violating Kansas restrictions on late-term abortions.

Roeder, the sole defense witness, testified Thursday that he considered elaborate schemes to stop the doctor, including chopping off his hands, crashing a car into him or sneaking into his home to kill him.

But in the end, Roeder told jurors, the easiest way was to walk into Tiller’s church, put a gun to the doctor’s forehead and pull the trigger.

‘‘Those children were in immediate danger if someone did not stop George Tiller,’’ Roeder told jurors.

But after hearing Roeder testify, District Judge Warren Wilbert ruled that his lawyers failed to show that Tiller posed an imminent threat and the jury could not consider a manslaughter verdict.

Roeder was also convicted of aggravated assault for threatening two church ushers who tried to stop Roeder from fleeing.