Williams, executed by lethal injection at San Quentin prison, was declared dead at 12:35 am (0335 GMT), she added.
Several thousand people gathered outside the prison south of San Francisco and raised their voices in anger when the news was announced.
"It's over, but it's not," said civil rights leader Jesse Jackson, one of several high-profile figures to have supported Williams in his quest to avoid execution.
"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.
While on death row he insisted that he was innocent of the murders but admitted being a founder of the brutal Crips gang that terrorized Los Angeles at the time of the killings.
On death row he gained celebrity as an anti-violence crusader. An author of books urging children to steer clear of gangs, Williams was nominated several times for the Nobel Peace Prize.
Hollywood stars and civil rights activists joined an international campaign to save Williams' life because of his activism. 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.
The crowd of Williams supporters swelled to several thousand after the US Supreme Court declined to stay the execution six hours before it took place, clogging the narrow road that leads to the prison.
Some supporters carried candles, others signs reading "Save Tookie" and "Love is the answer".
Hollywood celebrities, including actors Jamie Foxx and Danny Glover, rapper Snoop Dogg, African-American leaders and opponents of the death penalty have championed Williams' case.
"We mourn the loss of this man," actor Mike Farrell of the television series MASH told the gathering. "His memory will stay with us. A man of peace. A man of decency."
Farrell accused Schwarzenegger of "leaving him [Williams] twisting in the wind for 97 hours" by waiting until Monday to deny clemency.
Jackson compared Williams to Jesus Christ and slain black leader Malcolm X.
"He has turned his scars into stars," Jackson said. "Long live the legacy of Tookie. Let's vow to stop violence in all its forms, whether at San Quentin or Iraq."
Earlier, Williams' defenders had produced what they said was a new witness who could clear him.
According to court documents, a former prisoner has testified that sheriff's deputies fed incriminating evidence to a witness who implicated Williams at his 1981 trial, and forged Williams' handwriting to create bogus evidence.
"I specifically recall him telling me he was 'gonna put that nigger away' and 'that Shotgun Crip is gonna do time', referring to Mr. Williams," Gordon Bradbury Von Ellerman, 46, said in an affidavit.
Von Ellerman said that he was in a Los Angeles County jail cell with another prisoner, Roger Oglesby, who was given crime reports from deputies so that he could practice Williams' signature.
Lawyers said that the information was sent to Schwarzenegger's office before his decision was announced.
Williams was found guilty of shooting one man in the back with a shotgun while robbing a convenience store, and a Chinese couple and their daughter at point blank range in a second raid on their family-run motel.
He never apologized for the killings, contending he would "rather die than lie" by admitting to the murders.
© 2005 Agence France-Presse
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