The US state of Georgia executed Troy Davis on Wednesday despite high opposition and an international outcry due to considerable doubts about his guilt over the 1991 murder conviction.

An unusually lengthy deliberation by the US Supreme Court delayed the process for over four hours but the prison said Davis was pronounced dead after a lethal injection at 11:08pm local time after desperate efforts by his defence team failed to win a stay (1:08pm AEST).

Troy Davis, 42, was convicted of the fatal shooting of a police officer. Despite a plea for clemency from over a million people worldwide.

Davis had escaped previous trysts with death thrice in what has been a racially-charged and sensitive case in America's Deep South which has crossed the two decade mark. Davis has become the symbolic face of the efforts to disband the death penalty.

The family of the dead policeman Mark MacPhail continue to insist Davis was guilty. In fact, several of them were in the Jackson state prison to witness him die.

The last-ditch bid to stay Davis's execution had earlier been rejected twice at state level and the five-member Georgia Board of Pardons and Paroles on Tuesday turned down his bid for clemency.

His death was marked by unrelenting last-minute drama when Georgia officials delayed the execution by an excruciating 3½ hours as they awaited a final ruling by the US Supreme Court.

Davis had been about to be strapped to a gurney to be injected, as state witnesses assembled to view his execution, when the schedule was interrupted. However, the court ultimately denied him a reprieve.

Hundreds of supporters, gathered outside the jail in Jackson, about 60 kilometres south-east of Atlanta, fell into despair once the decision was known. A police presence was in place to control any eruption of anger.

The nation's highest court was Davis's last chance after Georgia's judiciary rejected last-minute appeals from his defence team earlier in the day.

It was the fourth time that an execution date had been set for Davis, who was convicted of the 1989 shooting of 27-year-old policeman Mark MacPhail. The off-duty officer was shot in the chest and face when he sought to intervene in an argument in the car park of a Savannah takeaway.

Three stays had been granted since 2007, with Davis on one occasion coming within 2½ hours of being executed.

The extraordinary legal case has put America's death penalty in an uncomfortable spotlight.

Hundreds of thousands of people worldwide, including some from Australia, pleaded for clemency in the wake of a series of court appeals and with seven of nine witnesses having recanted their original testimony, some claiming to have been coerced by police.

No weapon, DNA evidence or surveillance footage was found to link Davis to the crime. His defence team pointed the finger at a second man who had been with Davis on that evening and who later became the prosecution's star witness.