SAN QUENTIN, Calif., Dec. 13 -- Stanley "Tookie" Williams, a gang leader-turned-peace advocate whose cause drew worldwide attention, was executed in San Quentin's death chamber Tuesday morning after Gov. Arnold Schwarzenegger denied his last-ditch plea for clemency and the Supreme Court refused to block the execution.

Having exhausted his appeals, Williams, 51, who co-founded the notorious Crips gang, was killed via injection at 12:35 p.m. Pacific time (3:35 EST) for four murders committed during two separate robberies in Southern California in 1979.

Media witnesses to the execution said that Williams appeared to be composed throughout the procedure, looking up several times at his supporters and once staring down the media witnesses watching him behind glass walls. They said he shook his head as if saying "for shame" and complained to those administering the injection when they took 12 minutes to find a vein in his left arm.

He was quoted as saying to the medical technician, "You still can't find it?"

He shook his head, as if to say, "no, no, no," said one of the witnesses, and grimaced before he lost consciousness. Most of the news media witnesses who spoke at a news conference expressed concern about the slowness of the process, apparently caused by difficulty tapping into Williams' vein.

One of his supporters watching the execution raised a fist in the air. Another, on the way out, said "the state of California has executed an innocent man."

A relative of one of his victims observing the execution wept, according to a witness.

Throughout the night, growing numbers of Williams supporters, and a much smaller group of pro-death penalty advocates, had gathered in front of the prison to chant, wave signs, sing and light candles.

About 2,500 people stood vigil outside of San Quentin prison, the vast majority of them to show support for Williams and protest the death penalty. The crowd included actors Mike Farrell and Sean Penn, singer Joan Baez, who sang at a makeshift stage and longtime activist Angela Davis.

On several occasions, small groups of Williams' supporters and detractors clashed in verbal skirmishes that broke up without intervention from police.

The crowd grew so thick as execution time, 12:01, approached that California Highway Patrol closed the exit to San Quentin from Highway 101.

The burly Williams, who had maintained his innocence since his arrest, gained prominence speaking out against gang violence in a series of children's books and through writings appealing directly to gangs. His supporters have said that the evidence against him was weak, that his lawyers botched his trial, and that during his years on death row his conversion to an anti-gang advocate made him a strong candidate for mercy.

Williams was nominated for the Nobel Peace Prize five times, and appeals from winners of the prize, religious leaders and Hollywood celebrities neared a fever pitch in the past several weeks, leading to one of the most fervent debates on the death penalty in the United States in years.

"The first thing you learn from the Bible is about forgiveness," actor Jamie Foxx told CNN in criticizing Schwarzenegger's decision. Foxx portrayed Williams in "Redemption," a made-for-television movie.

But Schwarzenegger, in a five-page statement, said: "Williams' case has been thoroughly reviewed in the 24 years since his convictions and death sentence. In all, Williams' case has been the subject of at least eight substantive judicial opinions." The governor added that he saw "no reason to disturb the judicial decisions that uphold the jury's decisions that he is guilty of these four murders and should pay with his life."

No prisoner has been granted clemency in California since Gov. Ronald Reagan commuted the death sentence of a mentally retarded man in 1967. Williams will be the 12th person executed in the state since capital punishment was reinstated in 1977. He will also be the third death row inmate for whom Schwarzenegger has refused to grant a reprieve.

Williams's attorneys have issued a furious series of petitions in recent weeks. On Sunday, they had asked the California Supreme Court to call off the execution. Just before the governor announced his decision, the U.S. Court of Appeals for the 9th Circuit denied Williams's request for a reprieve. The court said that there was no "clear and convincing evidence of actual innocence." Late Monday, the U.S. Supreme Court also refused to block the execution.

Those who asked Schwarzenegger to spare Williams's life include Archbishop Desmond Tutu; NAACP President Bruce S. Gordon; Sister Helen Prejean, author of "Dead Man Walking"; and rap singer Snoop Dogg, who said Williams had inspired him to leave the Crips and reform his life. Civil rights activist Jesse L. Jackson led a march across the Golden Gate Bridge on Monday night in support of Williams.

State law enforcement officials, including the Correctional Peace Officers Association, one of the most powerful unions in the state, have been united in their condemnation of Williams and fervently appealed to Schwarzenegger to let the execution proceed.

Williams was arrested in connection with four homicides committed on Feb. 28 and March 11, 1979, in Southern California. On Feb. 28, Albert L. Owens, 26, a clerk at a 7-Eleven store in Pico Rivera, was shot in the head while lying facedown on the floor during a robbery. On the second date, Tsai-Shai Yang, 63, and her husband, Yen-I Yang, 76, were executed in similar fashion with their daughter, Yee-Chen Lin, 43, during a holdup at their motel in south Los Angeles.

Among the evidence against Williams was a shotgun shell discovered at the crime scene that was linked to a gun he had bought five years earlier. That weapon was found under the bed of two associates accused of killing their business partner. The murder charges against them were dropped after they testified that Williams had confessed to them.

In his statement, Schwarzenegger said: "The evidence demonstrating Williams is guilty of these murders is strong and compelling. Based on the cumulative weight of the evidence, there is no reason to second guess the jury's decision of guilt or raise significant doubts or serious reservations about Williams's conviction and death sentence."

Critics branded Schwarzenegger's decision as a safe political move that did not consider the possibility that California could impose a moratorium on state executions in the next few months.

"Schwarzenegger could have called for a moratorium today," said Assemblyman Mark Leno (D-San Francisco), who is co-sponsoring a bill that would impose a moratorium on executions until January 2009 to review the fairness of how the state imposes the death penalty. The bill was introduced in August and is scheduled to be heard in committee next month.

"It would be refreshing to see the state articulate the values of grace, mercy and redemption," Leno said. "Unfortunately, the governor has missed an opportunity to do just that."

Polls consistently show that a majority of Californians support the death penalty, and Schwarzenegger, who faces reelection next year, has been trying to recover from a disastrous special election in which the four measures he was pushing were defeated. He also has been under fire for the past several weeks from conservatives and others in the Republican Party for appointing a Democrat as his chief of staff.