ATLANTA -- A Georgia man convicted of killing an off-duty police officer was set to be executed Tuesday, even though witnesses have backed away from their testimony and questions remain about whether he is truly guilty.

Seven of the nine witnesses who helped put Troy Davis on death row for the 1989 murder have since recanted their testimony, and Davis' attorneys say that others claim another man pulled the trigger.

But prosecutors have labeled the witness statements "suspect" and the courts have consistently refused requests for a new trial.

Influential advocates, including former President Jimmy Carter and South Africa Archbishop Desmond Tutu, insist that there's enough doubt about his guilt to merit a new trial for 39-year-old Troy Davis. Davis' only hope for a reprieve lies with the U.S. Supreme Court.

A divided Georgia Supreme Court has twice rejected his request for a new trial, and rejected his appeal to delay the execution by a 6-1 vote Monday afternoon. The Georgia Board of Pardons and Paroles also turned down his bid for clemency after a daylong hearing.

As his legal options dwindle, Davis supporters have organized a series of rallies and vigils around the state.

They are calling for the prison staff scheduled to oversee the execution to call in sick and picketed the offices of the company that provides medical staff for the execution. They also plan a demonstration at the Georgia Capitol and vigils at eight towns around the state.

Amnesty International has taken up the cause, helping organize rallies as far away as Paris. The execution site, a state prison in Jackson, Ga., is expected to attract dozens of protesters, including the Rev. Al Sharpton.

Davis was convicted of the 1989 murder of 27-year-old officer Mark MacPhail, who was working off-duty as a security guard at a bus station.

MacPhail had rushed to help a homeless man who had been pistol-whipped at a nearby parking lot, and was shot twice when he approached Davis and two other men.

Witnesses identified Davis as the shooter, and at the 1991 trial, prosecutors said he wore a "smirk on his face" as he fired the gun.

But Davis' lawyers say new evidence proves their client was a victim of mistaken identity. Besides those who have recanted their testimony, three others who did not testify have said Sylvester "Red" Coles — who testified against Davis at his trial — confessed to the killing.

He refused to talk about the case when contacted by The Associated Press during a 2007 Chatham County court appearance and he has no listed phone number.

Prosecutors have contended in court hearings the case is closed. They also say some of the witness affidavits simply repeat what a trial jury has already heard, while others are irrelevant because they come from witnesses who never testified.

Davis' attorneys asked the U.S. Supreme Court to stay the execution until it has a chance to discuss whether to hear the case at a conference next week. The court could decide within hours.

"The world is watching Georgia," said Martina Correia, Davis' sister. "Everything you do in the dark always comes back to light."