The first man to face the death penalty in the United States for killing a doctor who performed abortions has been executed.
Hill did not appeal against his death sentence
Paul Hill, 49, died at 1808 local time (2208 GMT) in northern Florida, following his conviction for the murder of two people outside an abortion clinic in 1994.
He gave no resistance as the lethal injection was administered, a Florida department of corrections spokesman said.
There are fears that Hill's execution may lead to a backlash from anti-abortion campaigners against doctors and clinics, and Florida abortion clinics and police are on heightened alert for reprisals.
Florida Governor Jeb Bush, a Republican who is the brother of US President George W Bush, could have commuted the death sentence to life imprisonment, but said he would not be "bullied" into stopping the execution.
In a last statement before his execution, Hill thanked God for his family and urged anyone against abortion to do what they could to prevent it.
"Two of the last things that I would like to say - if you believe abortion is a lethal force, you should oppose the force and do what you have to to stop it. May God help you to protect the unborn as you would want them to be protected."
Among the 100 demonstrators who gathered outside Florida's Starke Prison, many hailed the former Presbyterian minister and father of three as a hero.
Some anti-abortion campaigners believe killing is justified
At the time of Hill's death, they stood and held hands. Many carried placards proclaiming that he is now a martyr for his cause.
Some 300 law enforcement officers were drafted in to stand guard outside the prison in case of trouble, but the execution passed off peacefully, the BBC's Fergal Parkinson reported from outside the prison.
The execution has been criticised by an unusual alliance of anti-abortion Christian protesters and anti-death penalty activists.
Hill's lawyer, Michael Hirsch, told the BBC before the execution that the state would be acting illegally if it went ahead with the procedure.
"The court order in this case says his execution cannot be carried out until the completion of his federal life sentence, so we think that the government is acting outside the bounds of law," Mr Hirsch said.
Before the execution, Hill said he would kill again to save the unborn and was looking forward to dying for his cause.
"I believe the state, by executing me, will be making me a
martyr," he told a news conference.
However, mainstream pro-life groups have distanced themselves from violent tactics.
Anti-death penalty activists, for their part, had urged the Florida governor to "stop the martyrdom of Paul Hill".
"Please commute Paul Hill's death sentence to life in prison without possibility of parole, because killing Paul Hill will actually encourage more murder," a group called Floridians for Alternatives to the Death Penalty wrote in an open letter to Jeb Bush.
Hill was convicted of murdering Dr John Britton and his driver James Barrett outside the Ladies Center in Pensacola, Florida, in 1994.
Hill gave himself up to police, saying that he had carried out the killings to prevent "innocent babies" being killed.