Williams, 51, died at 12.35am (0835GMT) on Tuesday after receiving a lethal injection at San Quentin State Prison, officials said.

Before the execution, he was "complacent, quiet and thoughtful", Corrections Department spokeswoman Terry Thornton said.

The case became the state's highest-profile execution in decades.

Hollywood stars and capital punishment foes argued that Williams's sentence should be commuted to life in prison because he had made amends by writing children's books about the dangers of gangs and violence.

In the days leading up to the execution, state and federal courts refused to reopen his case.

On Monday, California governor Arnold Schwarzenegger denied Williams's request for clemency, suggesting that his supposed change of heart was not genuine because he had not shown any real remorse for the countless killings committed by the Crips.

"Is Williams's redemption complete and sincere, or is it just a hollow promise?" Schwarzenegger wrote. "Without an apology and atonement for these senseless and brutal killings, there can be no redemption."

1981 killings

Williams was condemned in 1981 for shooting and killing convenience store clerk Albert Owens, 26, and killing Yen-I Yang, 76, Tsai-Shai Chen Yang, 63, and the couple's daughter Yu-Chin Yang Lin, 43, at the Los Angeles motel they owned. Williams said he was innocent of those murders.

Demonstrators asked California's
governor to grant clemency

Witnesses at the trial said Williams boasted about the killings, stating: "You should have heard the way he sounded when I shot him."

Williams is the 12th person executed in California since lawmakers reinstated the death penalty in 1977.

Some witnesses said the nurse who delivered the injection appeared to have trouble finding a vein in his muscular arm. At one point, he uttered something to the nurse and offered to help, said Steve Ornoski, the prison warden.

"He did seem frustrated," Ornoski said.

Demonstrators

About 1000 death penalty supporters and opponents gathered outside the prison to await the execution. Singer Joan Baez, actor Mike Farrell and the rights campaigner Reverend Jesse Jackson were among the celebrities who protested against the execution.

Said Baez: "Tonight is planned, efficient, calculated, antiseptic, cold-blooded murder and I think everyone who is here is here to try to enlist the morality and soul of this country."

A contingent of 40 people who had walked the 40km from San Francisco held signs calling for an end to "state-sponsored murder". Others said they wanted to honour the memory of Williams's victims.

Former Crips member Donald Archie, 51, was among those attending a candlelight vigil. He said he would work to spread Williams's anti-gang message.

"The work ain't going to stop," said Archie, who said he was known as "Sweetback" as a young Crips member.

"Tookie's body might lay down, but his spirit ain't going nowhere. I want everyone to know that, the spirit lives."