A last-ditch appeal was under way Monday to spare the life of American Troy Davis, who has already escaped three dates with death for gunning down an off-duty policeman in 1989. A Georgia parole board must decide whether Davis should be put to death by lethal injection on Wednesday or be granted clemency in a high-profile case that 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. The question marks over Davis\'s conviction -- there was no physical evidence linking Davis to the crime and seven of the nine witnesses have since withdrawn or changed their testimony -- have elevated the case to a cause celebre among death penalty opponents. Davis supporters say close to one million people across the globe have signed petitions calling for clemency, with petitions last week delivered to state authorities containing about 650,000 signatures.Some 300 rallies, vigils and events have taken place worldwide, including in New York, Washington, Peru, Paris and Oslo, among others, and nearly 200,000 of the signatures have been collected in the last 72 hours alone. However the family of the victim maintain that Davis, 42, was guilty of the shooting. \"We are the true victims here,\" said MacPhail\'s widow Joan outside the appeal board hearing in Atlanta, pledging that she and her two children would attend Davis\'s execution. MacPhail\'s daughter, Madison, 24, shedding tears, said: \"A future was taken from me. Things like my wedding. In three years I will be older than my father was. He is forever frozen at 27.\" The five-member parole board was deliberating after Monday\'s hearing and will later issue a simple majority decision in what is widely seen as the last chance for Davis. 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,\" said EU foreign policy chief Catherine Ashton, calling for his death sentence \"to be urgently commuted.\"In the latest pro-Davis protest, an estimated 150 to 200 demonstrators carried signs outside the closed-door hearing of the Georgia Board of Pardons and Paroles, reading, \"Justice, Free Troy Davis,\" and \"We are Troy Davis.\"Another placard read: \"Too much Doubt. Save Troy Davis.\" Stephen Marsh, a lawyer for Davis, told reporters during a break in the session he was \"thankful\" to the board for the hearing and that his client should be spared. \"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.\"Raphael Warnock, a senior pastor at Ebenezer Baptist Church who attended Monday\'s hearing, said one juror from the original trial was among the witnesses called by lawyers for Davis. Death penalty lawyer and Yale law professor Stephen Bright told the Atlanta Journal Constitution, a newspaper which has revealed doubts over the conviction: \"This case is extraordinary because there have been substantial questions of his innocence for almost a decade.\"It has attracted attention from all around the world, and the extraordinary number of people supporting him -- and the prominence of some of them -- is unprecedented.\" Davis has always maintained his innocence and critics say the case 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.The US Supreme Court became involved in 2009 and ordered a federal judge in Savannah to convene a hearing to consider new evidence. In August 2010, however, a US District Court in Georgia ruled that Davis had failed to prove his innocence and denied him a new trial. The top US court then turned down a subsequent appeal.Davis is scheduled to die by lethal injection at a prison in Jackson, Georgia, south of Atlanta on Wednesday.Among those calling for clemency are former US president Jimmy Carter, Pope Benedict XVI and US actress Susan Sarandon, as well as the National Association for the Advancement of Colored People, and Amnesty International.