Death row inmate Troy Davis has been executed in the US state of Georgia for the fatal shooting of policeman Mark MacPhail in 1989.
Davis' death was delayed for hours while the US Supreme Court considered an eleventh-hour appeal for clemency.
The 42-year-old's case was heavily disputed after most of the witnesses recanted or changed their testimony.
Inside the jail in Jackson, Georgia, Davis protested his innocence until the end as supporters protested outside.
There was a heavy police presence as hundreds held a vigil awaiting news from the US Supreme Court.
"I am innocent," Davis said moments before he was executed. "I did not have a gun."
Davis was convicted in 1991 of killing MacPhail, an off-duty police officer, but always maintained he was innocent.
The US Supreme Court judges took more than four hours to issue their rejection of the final appeal, an unusually long time for such a ruling.
"The application for stay of execution of sentence of death presented to Justice [Clarence] Thomas and by him referred to the Court is denied," it read.
Davis continued to protest his innocence in the death chamber.
"For those about to take my life, may God have mercy on your souls. May God bless your souls.
"All I can ask... is that you look deeper into this case so that you really can finally see the truth. I ask my family and friends to continue to fight this fight."
Davis was pronounced dead at 23:08 (03:08 GMT Thursday), 15 minutes after the lethal injection began.
MacPhail was shot dead in July 1989 as he tried to help a homeless man who was being attacked in a Burger King car park.
Prosecutors said Davis was beating the man with a gun after demanding a beer from him.
No gun was found and no DNA evidence conclusively linked Davis to the murder.
On Wednesday morning, Davis' lawyers appealed to the county court responsible for Georgia's death row, but that was also rejected.
The legal team had argued that ballistic testing from the case was flawed.
The pardons board also dismissed an appeal to reconsider their decision on Monday to deny Davis clemency.
Prosecutors said they had no doubts as to his guilt.
"I'm kind of numb. I can't believe that it's really happened," Anneliese MacPhail, the mother of the murdered policeman, told the Associated Press news agency after Davis was killed.
"All the feelings of relief and peace I've been waiting for all these years, they will come later. I certainly do want some peace."
Davis counted Pope Benedict XVI and former US President Jimmy Carter among his supporters, as well as US conservative figures like former member of the House of Representatives Bob Barr and former FBI director William Sessions.
Outside the prison, hundreds of people gathered chanting: "They say, death row; we say, hell no".
Around 10 counter-demonstrators were also present, voicing support for the death penalty and for the family of MacPhail.
There was a heavy police presence, including large numbers of riot police, but no disturbances were reported.
Davis' execution date had already been changed three times.
Protests had taken on an international dimension since Monday's decision to deny clemency by the Georgia pardons board.
The Council of Europe had also called for Davis' sentence to be commuted.
Amnesty International and other groups organised protests at the US embassy in Paris, where 150 people gathered in Place de la Concorde, holding signs bearing Davis' image.
"We strongly deplore that the numerous appeals for clemency were not heeded," the French foreign ministry said after the execution.
In Washington DC dozens gathered outside the White House, in the hope that President Barack Obama might intervene at the last-minute.
But White House press secretary Jay Carney said it would not be appropriate for the president to interfere in specific cases of state prosecution, such as this one.
Reports suggested around a dozen people were arrested for refusing to co-operate with police.
Meanwhile in the US state of Texas another death row inmate, Lawrence Russell Brewer, was executed on Wednesday evening - in a very different case.
In 1998, white supremacist gang member Brewer, 44, dragged a black man chained to the back of a pick-up truck along a road until he died.