Anti-death penalty campaigners have rallied for clemency along with Davis' supporters [Reuters]

A last-ditch appeal is under way in the US to stop the execution of Troy Davis for the 1989 murder of an off-duty police officer.

A Georgia parole board must decide whether Davis should be put to death by lethal injection on Wednesday or be granted clemency in a case which has garnered worldwide attention and twice reached the US Supreme Court.

Davis was convicted in 1991 of murdering Mark MacPhail, an off-duty policeman who was working as a security guard at a restaurant in Savannah when he intervened in an argument in a parking lot and was shot dead.

Davis, 42, has long claimed he is innocent. There was no physical evidence linking him to the crime and seven of the nine witnesses have since withdrawn or changed their testimony.

Davis supporters say close to one million people across the globe have signed petitions calling for clemency, nearly 200,000 of the signatures collected in the last 72 hours alone.

About 300 rallies, vigils and events have taken place worldwide, including in New York, Washington, Peru, France and Norway.

Inside the closed-door meeting of the Board of Pardons and Paroles, a parade of attorneys and supporters asked the panel to spare Davis' life.

Stephen Marsh, a lawyer for Davis, told reporters during a break in the session he was "thankful" to the board for the hearing.

"We believe we have established substantial doubt," he said.

"Given the level of doubt that exists in this case, we believe that an execution is simply not appropriate."

'Compelling doubts'

However the family of the victim maintain that Davis was guilty of the shooting.

"We are the true victims here," MacPhail's widow Joan said outside the appeal board hearing in Atlanta, pledging that she and her two children would attend Davis's execution.

Georgia has executed 51 people convicted of murder since 1976 and clemency has been granted on only seven occasions since. Davis was refused clemency by the parole board in September 2008 but some of its members have since changed.

The European Union on Monday urged justice officials against killing Davis.

"Serious and compelling doubts have persistently surrounded the evidence on which Mr Davis was convicted," EU foreign policy chief Catherine Ashton said, calling for his death sentence "to be urgently commuted."

Among those calling for clemency are former US President Jimmy Carter and Pope Benedict XVI, as well as the National Association for the Advancement of Coloured People and Amnesty International.

Critics say the case has highlighted the face of a corrupted justice system in the deep south, with a black man wrongly and hastily convicted of killing a white police officer.

MacPhail was shot to death in August 1989 after rushing to help Larry Young, a homeless man 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 shot the officer to death.

In June 2010, a court heard two witnesses who said they falsely incriminated Davis and two others who said another man had confessed to being the actual killer.

Source: Agencies