The southern US state of Georgia executed Troy Davis yesterday, despite doubts over his 1991 murder conviction that made him a poster child for the global movement to end the death penalty.

Former US president Jimmy Carter, Archbishop Desmond Tutu and Pope Benedict XVI had all weighed in on his behalf in a racially charged case that spanned two decades, becoming a cause celebre for death penalty opponents.

In dramatic scenes at the prison near Jackson, hundreds of protesters bearing placards thought Davis had earned an 11th-hour reprieve from the US Supreme Court and erupted in cheers, only to have their hopes dashed.

After an unusually long deliberation, which delayed proceedings by almost four hours, the highest court in the land issued a terse statement rejecting a stay of execution, snuffing out any last chance his life would be spared.

A lethal concoction of drugs began to be administered at 10.53pm local time and Davis was pronounced dead 15 minutes later.

His execution appeared to bring some relief to the family of slain policeman Mark MacPhail, but it incensed those who believed it was a miscarriage of justice. France and Germany led the international condemnation.

Davis had escaped three previous dates with death during more than 20 years of legal wrangling. His last moments were described afterwards for television cameras by local radio journalist John Lewis, who witnessed the event.

"He was strapped to the gurney when we walked in and when the warden asked if he had to make a statement, he lifted his head up and looked directly at the front row, right where the MacPhail family and friends were sitting.

"He said he did not have a gun. He said he was not the one who took their son, father, brother and he said he was innocent," Lewis said.

Questions are bound to linger over the case due to the lack of physical evidence tying Davis to the crime and the resulting reliance on eyewitness testimony, much of which was later changed or recanted.

But for the family of MacPhail - who was shot in the heart and the head as he intervened in an argument in a Burger King parking lot in Savannah, Georgia on August 19 1989 - it was justice served.

"We have put up with this stuff for 22 years, and it is time for justice today," widow Joan MacPhail-Harris said before their son, who was an infant at the time of the murder, watched Davis die along with the victim's brother.

"They spent the entire time just staring at Troy Davis, never turned their heads, never did anything but stare ahead," said Lewis.

"As they were leaving, they hugged somebody and they seemed to smile about it. So, for the MacPhail family at least, they seemed to get some satisfaction from what happened."

Whether Davis was guilty or not, it was the perceived weakness of the case that brought an angry stream of reactions from Europe.

"We strongly deplore that the numerous appeals for clemency were not heeded," the French foreign ministry said.

"There are still serious doubts about his guilt," said Germany's junior minister for human rights Markus Loening.

"An execution is irreversible - a judicial error can never be repaired."

Larry Cox, executive director of rights group Amnesty International, said the US justice system had been "shaken to its core" as Georgia had executed a man "who may well be innocent".

"Killing a man under this enormous cloud of doubt is horrific, and amounts to a catastrophic failure of the justice system," he said, as activists hoped this would be a turning point to steer the US away from capital punishment.

In last-gasp motions, Davis's lawyers had cited new ballistics evidence and alleged misleading testimony.

But there was no substantial new evidence and the legal power really rested with a panel that had already denied Davis clemency twice, most recently on Tuesday.

The five-member Georgia Board of Pardons and Paroles refused yesterday to reconsider that decision and also turned down a request to allow Davis to take a polygraph test to prove his innocence.

Lawyers refused to give up, seeking a last-minute stay of execution and petitioning the local courts in a 40-page filing.

Hope grew among Davis's supporters after the matter went all the way to the US Supreme Court and deliberations dragged into a fourth hour, to the clear bemusement of legal experts.

But the crowds at the prison in Jackson and outside the Supreme Court in Washington were left stunned when the rejection came.

The case has been plagued by questions of racial bias since 27-year-old MacPhail was white and Davis, who was 20 at the time of the murder and has always maintained his innocence, is black.

He became the 35th person to be executed this year in the US . Thirty-four of the 50 US states still have the death penalty.