ATLANTA (AP) — Troy Davis supporters in the U.S. and Europe were trying just about anything to spare him from lethal injection Wednesday evening for killing an off-duty Georgia policeman, a crime he and others have insisted for years that he did not commit.

Supporters planned vigils around the world. They’ll be outside Georgia’s death row prison in Jackson and at U.S. embassies in Europe.

The 42-year-old’s most realistic, though slim, chance for reprieve is through the courts, and his lawyers are trying. His backers have tried increasingly frenzied measures: offering for Davis to take a polygraph test, urging prison workers to strike or call in sick, posting a judge’s phone number online, urging people to call and ask him to put a stop to the 7 p.m. execution. They’ve even considered a desperate appeal for White House intervention.

“We’re trying everything we can do, everything under the law,” said Chester Dunham, a civil rights activist and talk show host protesting in Savannah, where in 1989, prosecutors say Davis fatally shot 27-year-old Mark MacPhail.

Davis’ supporters include former President Jimmy Carter, Pope Benedict XVI and a former FBI director, the NAACP, as well as conservative figures. The U.S. Supreme Court even gave him an unusual opportunity to prove his innocence last year, but ultimately didn’t hear the merits of the case.

Several witnesses have recanted their accounts that it was Davis who pulled the trigger, and some jurors have said they’ve changed their minds about his guilt. Still, prosecutors and MacPhail’s family have staunchly backed the verdict and state and federal courts have repeatedly upheld his conviction.

MacPhail was working security at a bus station on Aug. 19, 1989, and rushed to the aid of Larry Young, a homeless man who prosecutors say Davis was bashing with a handgun after asking him for a beer. When MacPhail got there, they say Davis had a smirk on his face as he shot the officer to death in a Burger King parking lot. Others have claimed the man with Davis that night has told people he actually shot the officer.

No gun was ever found, but shell casings were linked, prosecutors say, to an earlier shooting for which Davis was convicted. Witnesses placed Davis at the crime scene and identified him as the shooter. However, no other physical evidence was found, including blood or DNA, that tied Davis to the crime.

As time ticked toward the execution, an upbeat and prayerful Davis turned down an offer for a special last meal and planned to spend his final hours meeting with friends, family and supporters. Meanwhile, two attempts to prove his innocence were rejected: a polygraph test and another hearing before the pardons board.

His attorney Stephen Marsh said Davis would only submit to a polygraph test if pardons officials would take it seriously.

“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.

His lawyers, meanwhile, are trying the legal avenues left to them, filing a motion in a county court challenging the ballistics evidence and eyewitness testimony. A judge could at least delay the execution, which has happened three times before. Most believe arguments on the merits of the case have been exhausted, however.

The National Association for the Advancement of Colored People, which has helped lead the charge to stop the execution, said it was considering asking President Barack Obama to intervene. Obama cannot grant Davis clemency since it was a state conviction, but could potentially halt the execution by asking for an investigation into a federal issue if one exists, though that was unlikely, said Richard Dieter, executive director of the Death Penalty Information Center.

In Savannah, 16 Davis supporters gathered at the Chatham County courthouse to press District Attorney Larry Chisolm to help stop Davis’ execution. They said 240,000 people had signed petitions urging the state to spare Davis’ life, and delivered them in three large boxes to Chisolm’s courthouse office where they were received by a member of the prosecutor’s staff. Chisolm has said he’s powerless to intervene, but activists say they believe he has enough influence as district attorney to sway the outcome.

As for the new and changed accounts by some witnesses, an unmoved federal judge dismissed them during a hearing set up by the U.S. Supreme Court in 2010. He said while the “new evidence casts some additional, minimal doubt on his conviction, it is largely smoke and mirrors.”

It was the first time in 50 years that justices had considered a request to grant a new trial for a death row inmate. It set a tough standard for Davis to exonerate himself, ruling his attorneys must “clearly establish” Davis’ innocence — a higher bar to meet than prosecutors having to prove guilt.

Once the hearing judge made his ruling, the justices didn’t take up the case.

Prosecutors say they 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.”

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.”

Spencer Lawton, the district attorney who secured Davis’ conviction in 1991, however, 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.”

The latest motion filed in Butts County Court on Wednesday disputes testimony from a Georgia Bureau of Investigation expert at Davis’ 1991 trial. It claims his testimony is no longer reliable that shell casings found at the scene of MacPhail’s murder were linked to those found at the scene of another shooting for which Davis was convicted.

It also challenged testimony from Harriet Murray, an eyewitness who at trial identified Davis as the shooter. And it said evidence from another witness, Kevin McQueen, who said Davis confessed the killing to him is “patently false” and unreliable.

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Copyright 2011 The Associated Press.