ATLANTA (AP) — With less than half a day left to live, death row inmate Troy Davis faced execution Wednesday despite a furious campaign in the U.S. and Europe to win clemency for the 1989 slaying of a Georgia policeman he claims he did not commit.

Supporters planned vigils outside Georgia’s death row prison in Jackson and protests at U.S. embassies in Europe. Davis’ attorneys lost a bid to give him a polygraph test to prove his innocence but planned another late appeal, this one aimed at blocking the execution by convincing a judge that some of the original evidence was questionable.

Defense lawyer Stephen Marsh told The Associated Press that the Georgia Department of Corrections denied his request to allow Davis to take a polygraph test. Marsh had said he hoped the polygraph would convince the state pardons board to reconsider a decision against clemency.

After winning three delays since 2007, Davis lost his most realistic chance at last-minute clemency this week when the state pardons board denied his request. He was set to be executed by injection at 7 p.m. (2300 GMT) Wednesday for the 1989 killing of Mark MacPhail, an off-duty police officer who was working as a security guard in Savannah when he was shot dead rushing to help a homeless man who had been attacked.

Some witnesses who fingered him at trial as the shooter later recanted, and others who did not testify came forward to say another man did it. But a federal judge dismissed those changed and new accounts as “largely smoke and mirrors” after a hearing Davis was granted last year to argue for a new trial, which he did not win.

Davis didn’t want a last meal. He planned to spend his final hours meeting with friends, family and supporters. According to an advocate who met with him late Tuesday, he was upbeat, prayerful and expected last-minute wrangling by attorneys.

Attorney Stephen Marsh said he had asked state prisons officials and the pardons board if they would allow a polygraph test. A prisons spokeswoman said she was unaware of the request and the pardons board didn’t immediately respond.

“He doesn’t want to spend three hours away from his family on what could be the last day of his life if it won’t make any difference,” Marsh said.

Davis has received support from hundreds of thousands of people, including a former FBI director, former President Jimmy Carter and Pope Benedict XVI. Some of his backers resorted to urging prison workers to strike or call in sick Wednesday, and they considered a desperate appeal for White House intervention. And some of Davis’ supporters were considering whether to ask President Barack Obama to intervene, a move that legal experts said was unlikely.

In Europe, where the planned execution has drawn widespread criticism, politicians and activists were making a last-minute appeal to the state of Georgia to refrain from executing Davis. Amnesty International and other groups planned a protest outside the U.S. Embassy in Paris later Wednesday and Amnesty also called a vigil outside the U.S. Embassy in London.

Parliamentarians and government ministers from the Council of Europe, the continent’s human rights watchdog, called for Davis’ sentence to be commuted. Renate Wohlwend of the Council’s Parliamentary Assembly said that “to carry out this irrevocable act now would be a terrible mistake which could lead to a tragic injustice.”

The U.S. Supreme Court gave him an unusual opportunity to prove his innocence last year, but his attorneys failed to convince a judge he didn’t do it. State and federal courts have repeatedly upheld his conviction.

Prosecutors have no doubt they charged the right person, and MacPhail’s family lobbied the pardons board Monday to reject Davis’ clemency appeal. The board refused to stop the execution a day later.

“He has had ample time to prove his innocence,” said MacPhail’s widow, Joan MacPhail-Harris. “And he is not innocent.”

Spencer Lawton, the district attorney who secured Davis’ conviction in 1991, said he was embarrassed for the judicial system that the execution has taken so long.

“What we have had is a manufactured appearance of doubt which has taken on the quality of legitimate doubt itself. And all of it is exquisitely unfair,” said Lawton, who retired as Chatham County’s head prosecutor in 2008. “The good news is we live in a civilized society where questions like this are decided based on fact in open and transparent courts of law, and not on street corners.”

Davis supporters said they will push the pardons board to reconsider his case. They also asked Savannah prosecutors to block the execution, although Chatham County District Attorney Larry Chisolm said in a statement he was powerless to withdraw an execution order for Davis issued by a state Superior Court judge.

“We appreciate the outpouring of interest in this case; however, this matter is beyond our control,” Chisolm said.

Davis’ lawyers drew up a late appeal asking a local judge to block the execution over evidence they object to. Defense attorney Brian Kammer told The Associated Press he would file the appeal in Superior Court in Butts County, home of the state’s death row, when it opens Wednesday.

The motion disputes testimony from a ballistics examiner who claimed that the bullets fired in a previous shooting that Davis was convicted of may have come from the same gun that fired at MacPhail. And it challenged eyewitness testimony from Harriet Murray, a witness who claimed at the trial to have identified Davis as the shooter.

It asks the court to vacate Davis’ execution, or at least delay it by 90 days, on grounds that it was “based on false, misleading and materially inaccurate evidence.”

MacPhail was shot to death Aug. 19, 1989, after coming to the aid of Larry Young, who was pistol-whipped in a Burger King parking lot. Prosecutors say Davis was with another man who was demanding that Young give him a beer when Davis pulled out a handgun and bashed Young with it. When MacPhail arrived to help, they say Davis had a smirk on his face as he shot the officer to death.

Witnesses placed Davis at the crime scene and identified him as the shooter. Shell casings were linked to an earlier shooting that Davis was convicted of. There was no other physical evidence. No blood or DNA tied Davis to the crime and the weapon was never found.

Davis’ attorneys say seven of nine key witnesses who testified at his trial have disputed all or parts of their testimony.

The state initially planned to execute him in July 2007 but the pardons board granted him a stay less than 24 hours before he was to die. The U.S. Supreme Court stepped in a year later and halted the lethal injection two hours before he was to be executed. And a federal appeals court halted another planned execution a few months later.

Over the years, Davis has picked up high-profile support from a host of dignitaries and dozens of federal lawmakers. Conservative figures have also advocated on his behalf, including former U.S. Rep. Bob Barr, ex-Justice Department official Larry Thompson and one-time FBI Director William Sessions.

The U.S. Supreme Court granted Davis a hearing to prove his innocence, the first time it had done so for a death row inmate in at least 50 years. At that June 2010 hearing, two witnesses testified that they falsely incriminated Davis at his trial when they said Davis confessed to the killing. Two others told the judge the man with Davis that night later said he shot MacPhail.

Prosecutors, though, argued that Davis’ lawyers were simply rehashing old testimony that had already been rejected by a jury. And they said no trial court could ever consider the hearsay from the other witnesses who blamed the other man for the crime.

U.S. District Judge William T. Moore Jr. sided with them. He said the evidence presented at the hearing wasn’t nearly enough to prove Davis is innocent and validate his request for a new trial. He said while Davis’ “new evidence casts some additional, minimal doubt on his conviction, it is largely smoke and mirrors.”

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Copyright 2011 The Associated Press.