Troy Davis, sentenced to death by the state of Georgia for the 1989 murder of an off-duty police officer, is due to be executed Wednesday night.

Protests and final appeals by lawyers are continuing, but time is running out for the death row inmate, the Atlanta Journal-Constitution reports. Davis is scheduled to die by lethal injection at 7 p.m.

Earlier Wednesday, officials at Georgia Diagnostic Prison, where Davis is being held, turned away lawyers who wanted to give Davis a polygraph test.  

The Atlanta Journal-Constitution reports:

Davis' final chances appear to rest on a court petition filed this morning before a Superior Court judge in Butts County, home to Georgia's death row. The filing says ballistics evidence used to convict Davis during the 1991 trial has been discredited as has testimony from a woman who said she was an eyewitness to the murder and a jailhouse snitch who testified at trial that Davis told him he was the killer.

Davis, 42, has on three previous occasions faced an execution date set by the state of Georgia, only to have it stayed, once with just hours to spare. 

But on Tuesday, Davis lost a final appeal for clemency from the Georgia Board of Pardons and Paroles.

More from GlobalPost: Death row inmate Troy Davis denied clemency in Georgia

Supporters in the United States and around the world have rallied to try and save Davis' life, and are planning international protests and vigils, including outside Georgia's death row prison in Jackson, CBS News reports.

Celebrities have also joined in the call for clemency. P. Diddy, Kim Kardashian and John Legend are among those celebrities who used Twitter to voice their support for Davis, Fox News in Atlanta reports.

Actress and activist Mia Farrow tweeted that “GA prison medical staff shd stay home tomorrow.Grievous errors in case against #TroyDavis. #TooMuchDoubt for execution to be carried out.” 

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

Since the conviction, many of the witnesses have changed their testimony, and there is no clear physical evidence linking Davis to the scene.

His lawyers have argued that a lack of physical evidence and dubious witness testimony provided enough doubt about his guilt to spare him the death penalty, The New York Times says.

But MacPhail's relatives said they were relieved at the Georgia State Board of Pardons and Paroles decision to deny clemency to Davis, the BBC reports.

"Justice was finally served for my father," said Mark MacPhail Jr, the victim's son, according to the BBC. "The truth was finally heard."

A last-ditch international campaign to save Davis saw more than 630,000 letters delivered to the Georgia pardons board by Amnesty International, the Times reports. Among the people who asked that the board give him clemency were former U.S. President Jimmy Carter, Archbishop Emeritus Desmond Tutu, singer Cee Lo Green and Pope Benedict XVI.

Since 1976 Georgia has executed 51 people convicted of murder, with clemency granted on only seven occasions.