United States authorities on Tuesday executed Stanley "Tookie" Williams, a convicted killer who was at the centre of one of biggest anti-death penalty campaigns in the United States in decades, a spokesperson for San Quentin prison said.

Williams, executed by lethal injection, was declared dead at 12:35, she added.

Several thousand people gathered outside the prison, on the shores of the Pacific Ocean south of San Francisco, raising their voices in anger when Williams' execution was announced.

"It's over, but it's not," said Reverend Jesse Jackson, one of several well known personalities who supported Williams in his quest to have his execution stayed.

"He came in without any kind of resistance, was strapped down, showed no kind of resistance whatsoever," said Los Angeles Times reporter Steve Lopez, who witnessed the execution along with nearly 40 other people, including supporters of Williams and the families of his victims.

Williams, 51, was found guilty in 1981 of four murders, those of a convenience store clerk and a family of Chinese immigrants.

He had admitted being a founder of the brutal Crips gang that terrorised Los Angeles at the time but denied the killings. While on death row he renounced his violent past and wrote acclaimed books to try to persuade youths not to join gangs.

Hollywood stars and civil rights activists had joined an international campaign to try to save Williams' life. But all court appeals were rejected and California governor Arnold Schwarzenegger turned down his clemency bid on Monday.

"Clemency cases are always difficult and this one is no exception," Schwarzenegger said in a statement.

"After studying the evidence, searching the history, listening to the arguments and wrestling with the profound consequences, I could find no justification for granting clemency.

"The facts do not justify overturning the jury's verdict or the decisions of the courts in this case," the governor added.

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