Defiant to the end, he told relatives of Mark MacPhail that his 1989 slaying was not his fault. "I did not have a gun," he insisted.
"For those about to take my life," he told prison officials, "may God have mercy on your souls. May God bless your souls."
Davis was declared dead at 11:08. The lethal injection began about 15 minutes earlier, after the Supreme Court rejected an 11th-hour request for a stay.
The court did not comment on its order, which came about four hours after it received the request and more than three hours after the planned execution time.
Though Davis' attorneys said seven of nine key witnesses against him disputed all or parts of their testimony, state and federal judges repeatedly ruled against granting him a new trial. As the court losses piled up Wednesday, his offer to take a polygraph test was rejected and the pardons board refused to give him one more hearing.
Davis' supporters staged vigils in the U.S. and Europe, declaring "I am Troy Davis" on signs, T-shirts and the Internet. Some tried increasingly frenzied measures, urging prison workers to stay home and even posting a judge's phone number online, hoping people will press him to put a stop to the lethal injection.
President Barack Obama deflected calls for him to get involved.
"They say death row; we say hell no!" protesters shouted outside the Jackson prison where Davis was to be executed. In Washington, a crowd outside the Supreme Court yelled the same chant.
As many as 700 demonstrators gathered outside the prison as a few dozen riot police stood watch, but the crowd thinned as the night wore on and the outcome became clear. The scene turned eerily quiet as word of the high court's decision spread, with demonstrators hugging, crying, praying, holding candles and gathering around Davis' family.
Laura Moye of Amnesty International said the execution would be "the best argument for abolishing the death penalty."
"The state of Georgia is about to demonstrate why government can't be trusted with the power over life and death," she said.
About 10 counterdemonstrators also were outside the prison, showing support for the death penalty and the family of Mark MacPhail, the man Davis was convicted of killing in 1989. MacPhail's son and brother attended the execution.
"He had all the chances in the world," his mother, Anneliese MacPhail, said of Davis in a telephone interview. "It has got to come to an end."
At a Paris rally, many of the roughly 150 demonstrators carried signs emblazoned with Davis' face. "Everyone who looks a little bit at the case knows that there is too much doubt to execute him," Nicolas Krameyer of Amnesty International said at the protest.
Davis' execution has been stopped three times since 2007, but on Wednesday the 42-year-old ran out of legal options.
As his last hours ticked away, an upbeat and prayerful Davis turned down an offer for a special last meal as he met with friends, family and supporters.
"Troy Davis has impacted the world," his sister Martina Correia said at a news conference. "They say, `I am Troy Davis,' in languages he can't speak."
His attorney Stephen Marsh said Davis would have spent part of Wednesday taking a polygraph test if pardons officials had taken his offer seriously.
"He doesn't want to spend three hours away from his family on what could be the last day of his life if it won't make any difference," Marsh said.
Amnesty International says nearly 1 million people have signed a petition on Davis' behalf. His supporters include former President Jimmy Carter, Pope Benedict XVI, a former FBI director, the NAACP, several conservative figures and many celebrities, including hip-hop star Sean "P. Diddy" Combs.
"I'm trying to bring the word to the young people: There is too much doubt," rapper Big Boi, of the Atlanta-based group Outkast, said at a church near the prison.
The U.S. Supreme Court gave Davis an unusual opportunity to prove his innocence in a lower court last year, though the high court itself did not hear the merits of the case.
He was convicted in 1991 of killing MacPhail, who was working as a security guard at the time. MacPhail rushed to the aid of a homeless man who prosecutors said Davis was bashing with a handgun after asking him for a beer. Prosecutors said Davis had a smirk on his face as he shot the officer to death in a Burger King parking lot in Savannah.
No gun was ever found, but prosecutors say shell casings were linked to an earlier shooting for which Davis was convicted.
Witnesses placed Davis at the crime scene and identified him as the shooter, but several of them have recanted their accounts and some jurors have said they've changed their minds about his guilt. Others have claimed a man who was with Davis that night has told people he actually shot the officer.
"Such incredibly flawed eyewitness testimony should never be the basis for an execution," Marsh said. "To execute someone under these circumstances would be unconscionable."
State and federal courts, however, have repeatedly upheld Davis' conviction. One federal judge dismissed the evidence advanced by Davis' lawyers as "largely smoke and mirrors."
"He has had ample time to prove his innocence," said MacPhail's widow, Joan MacPhail-Harris. "And he is not innocent."
The last motion filed by Davis' attorneys in Butts County Court challenged testimony from two witnesses and disputed testimony from the expert who linked the shell casings to the earlier shooting involving Davis. Superior Court Judge Thomas Wilson and the Georgia Supreme Court rejected the appeal, and prosecutors said the filing was just a delay tactic.
The National Association for the Advancement of Colored People, which helped lead the charge to stop the execution, said it considered asking Obama to intervene, even though he cannot grant Davis clemency for a state conviction.
Press secretary Jay Carney issued a statement saying that although Obama "has worked to ensure accuracy and fairness in the criminal justice system," it was not appropriate for him "to weigh in on specific cases like this one, which is a state prosecution."
Dozens of protesters outside the White House called on the president to step in, and about 12 were arrested for disobeying police orders.
Davis was not the only U.S. inmate put to death Wednesday evening. In Texas, white supremacist gang member Lawrence Russell Brewer was put to death for the 1998 dragging death of a black man, James Byrd Jr., one of the most notorious hate crime murders in recent U.S. history.
Davis' best chance may have come last year, in a hearing ordered by the U.S. Supreme Court. It was the first time in 50 years that justices had considered a request to grant a new trial for a death row inmate.
The high court set a tough standard for Davis to exonerate himself, ruling that his attorneys must "clearly establish" Davis' innocence - a higher bar to meet than prosecutors having to prove guilt. After the hearing judge ruled in prosecutors' favor, the justices didn't take up the case.
The execution drew widespread criticism in Europe, where politicians and activists made last-minute pleas for a stay. Spencer Lawton, the district attorney who secured Davis' conviction in 1991, said he was embarrassed for the judicial system - not because of the execution, but because it took so long to carry out.
"What we have had is a manufactured appearance of doubt which has taken on the quality of legitimate doubt itself. And all of it is exquisitely unfair," said Lawton, who retired as Chatham County's head prosecutor in 2008. "The good news is we live in a civilized society where questions like this are decided based on fact in open and transparent courts of law, and not on street corners."
Associated Press reporters Russ Bynum in Savannah, Kate Brumback and Marina Hutchinson in Jackson, Eric Tucker and Erica Werner in Washington and Sohrab Monemi in Paris contributed to this report.