AFP - Troy Davis, an American convicted two decades ago of killing an off-duty policeman, has urged opponents of the death penalty to fight on after he is executed Wednesday following a failed bid for clemency.

The Georgia State Board of Pardons and Paroles ruled Tuesday against Davis, who is black, and refused to commute his sentence for the shooting in 1989 of Mark MacPhail, a married white father of a two-year-old girl and an infant boy.

The campaign to spare Davis's life drew high-profile support from former US president Jimmy Carter and Pope Benedict XVI, helping him escape three previous dates with death in a racially-charged case.

"The struggle for justice doesn't end with me," Davis said in a letter to supporters released to the public via Amnesty International USA after his legal appeal was refused.

"This struggle is for all the Troy Davises who came before me and all the ones who will come after me," he said in the message.

"I'm in good spirits and I'm prayerful and at peace. But I will not stop fighting until I've taken my last breath."

There was no physical evidence tying Davis, then 20 years old, to the shooting and several witnesses at his trial later recanted their testimony.

MacPhail, 27, had been working nights as a security guard when he intervened in a brawl in a Burger King parking lot in Savannah, Georgia and was shot in the heart and the head at point-blank range.

Some 2,000 protesters gathered, at Amnesty's urging, at the Georgia state capitol at 7:00 pm (2300 GMT) Tuesday, exactly 24 hours before Davis is due to become the 34th person executed in the United States this year.

The family of the victim have long maintained that Davis was guilty and that the execution should go ahead, with MacPhail's daughter telling journalists emotionally how she had been robbed of her father.

Davis, now 42, has always maintained his innocence amid doubts over his conviction and says the state of Georgia is about to execute an innocent man, but justice officials refused to commute his sentence.

"The board has considered the totality of the information presented in this case and thoroughly deliberated on it, after which the decision was to deny clemency," said a written statement. It did not disclose the vote breakdown. "We've been here three times before," said Anneliese MacPhail, the mother of the slain police officer. "We are ready to close this book and start our lives. This has been a long haul."

MacPhail's daughter Madison, now 23, choked back the tears after Monday's parole board hearing.

"The death penalty is the correct source of justice," she said.

All avenues for Davis now appear exhausted as Georgia's governor does not have the power to stay executions and experts said any last-minute filings to the state courts or the US Supreme Court would likely prove unsuccessful.

"I am utterly shocked and disappointed at the failure of our justice system at all levels to correct a miscarriage of justice," Davis attorney Brian Kammer said, as rights groups and activists rushed to condemn the decision.

African American leaders condemned the parole board's decision as emblematic of a US criminal justice system riven with racial inequality.

"This is Jim Crow in a new era," declared Reverend Raphael Warnock of the Ebenezer Baptist Church, referring to American segregation laws overruled by the 1964 Civil Rights Act.

The American Civil Liberties Union urged Georgia's prison workers to strike in a desperate bid to deprive the state of the wherewithal to carry out the execution.

Richard Dieter, executive director of the Death Penalty Information Center, said there was next-to-no chance Davis could earn a reprieve in what he called the "biggest capital punishment case in at least a decade."

The Supreme Court became involved in the Troy Davis case in 2009 and ordered a federal judge in Savannah to convene a hearing to consider new evidence.

In August 2010, however, a US District Court in Georgia ruled that Davis had failed to prove his innocence and denied him a new trial. The top US court turned down a subsequent appeal.