Last-ditch efforts were underway Monday to spare the life of a 38-year-old black man condemned to death for the murder of a white police officer, even though most of the witnesses have admitted lying under police duress.

Troy Davis was scheduled to be executed by lethal injection late Tuesday for his conviction in the August 1989 fatal shooting of a police officer.

A Georgia parole board on Monday was to consider Davis' appeal for clemency, after several witnesses recanted court testimony fingering Davis as the triggerman.

But prosecution lawyers have discounted the recantations, and are urging the court to go ahead with the 7:00 pm (1900 GMT) execution Tuesday at the state prison in Jackson, Georgia.

Davis was sentenced to death in 1991 for the fatal shooting of off-duty police officer Mark McPhails, 27, who was killed when he tried to break up a melee in the parking lot of a fast food restaurant where he was moonlighting as a security guard.

Twenty years old at the time of the shooting, Davis, who was singled out by several witnesses as the triggerman, told investigators that although he had been at the crime scene, he had no role in the killing.

In the years since his conviction, seven of nine witnesses have changed their stories, and now insist Davis was not the gunman. They said earlier statements implicating him had been coerced by strongarm police tactics.

Despite this new evidence that could serve to exonerate him, no state and federal courts have been willing to rehear Davis's case. A petition for a US Supreme Court appeal was denied in June.

Human rights advocates, who have urged Georgia's parole board to spare Davis's life, have had harsh criticism for a criminal justice system unwilling to hear new evidence that could exonerate death row inmates who possibly have been falsely convicted.

"It is deeply troubling to me that Georgia might proceed with this execution, given the strong claims of innocence in this case," said Sister Helen Prejean, who gained international fame as the compassionate Catholic nun who counseled a death row inmate in the state of Louisiana in the book and film "Dead Man Walking."

"We must confront the unalterable fact that the system of capital punishment is fallible, given that it is administered by fallible human beings," she said.

Prejean called Davis's inability to have his conviction reviewed in court a "textbook case" for capital punishment reform -- if not an outright ban.

Amnesty International also is lobbying on Davis's behalf.

"At times there are cases that are emblematic of the dysfunctional application of justice in this country," said Larry Cox, executive director of Amnesty International's US chapter.

"By refusing to review serious claims of innocence, the Supreme Court has revealed catastrophic flaws in the US death penalty machine," Cox said.

Said Sue Gunawardena-Vaughn, director of Amnesty International's Program to Abolish the Death Penalty: "It is appalling that so many judges were able to look away from such a grave breach of justice.

"Evidence of innocence simply hasn't mattered," in this and many other disputed death penalty cases, she said.

South African Nobel laureate Archbishop Desmond Tutu also has spoken out against the planned execution, while the Council of Europe Monday urged the United States not to commit what it said would be a error it would come to regret.

"The execution of Troy Davis ... could become famous as an irreversible mistake and a tragic miscarriage of justice," read a council statement Monday, adding that since Davis's conviction 16 years ago "all the developments related one way or another to his case cast more and more doubt on the validity of the judgment."