Stanley "Tookie" Williams, the co-founder of the notorious Crips street gang, whose apparent rehabilitation in prison opened a bitter debate over the death penalty in America, was executed this morning in California.

The 51-year-old died by lethal injection at 12.35am local time (8.35GMT) at San Quentin prison in front of about 50 witnesses, including relatives of the four people he killed in 1979. He asked his own family not to attend.

Williams was denied a stay of execution by Arnold Schwarzenegger, the California Governor, and the Supreme Court yesterday as he exhausted his last appeals. Mr Schwarzenegger refused clemency because Williams has never apologised for four murders he committed during the summer of 1979.

Rather, Williams's appeal was based on his much-storied redemption, which inspired a television film starring Jamie Foxx. During his 24 years in prison, the former gang leader wrote children’s books with anti-violence messages, negotiated truces between street gangs and was nominated five times for the Nobel Peace Prize.

Williams was convicted in 1981 for the murders of Albert Owens, a 7-Eleven convenience store clerk, and Yen-I Yang, Tsai-Shai Chen Yang, and the couple’s daughter Yu-Chin Yang Lin, at the Los Angeles motel they owned. He always claimed he was innocent.

But witnesses at his trial said Williams boasted about the brutal killings, stating: "You should have heard the way he sounded when I shot him" before growling and laughing for five to six minutes. Mr Schwarzenegger quoted from the trial transcript to support his decision.

"Is Williams’s redemption complete and sincere, or is it just a hollow promise?" Mr Schwarzenegger wrote. "Without an apology and atonement for these senseless and brutal killings, there can be no redemption. After studying the evidence, I could find no justification for granting clemency."

Just before the governor announced his decision, the 9th US Circuit of Appeals also denied Williams’s request for a reprieve, saying among other things that there was no "clear and convincing evidence of actual innocence".

Protests against Williams's sentence continued until his death. In the hours leading up to midnight, as Williams exhausted his legal options, around 1,000 protesters gathered outside San Quentin. Some carried candles, others signs reading "Save Tookie" and "Love is the answer".

Joan Baez, the country singer, was one of the protesters who made the pilgrimage up the narrow road leading to the institution on the shores of the Pacific Ocean, 30 miles south of San Francisco. As night fell she sang "Swing Low, Sweet Chariot" on a make-shift platform.

In downtown Los Angeles, where city leaders feared that the death of Williams could provoke the kind of unrest that followed the 1992 acquittal of police officers involved in the beating of Rodney King, the streets remained quiet.

Earlier, Raymond "The Hatchet Man" Locket, a member of the Westside Harlem Crips and a former associate of Williams, told LA Weekly: "Took die, the city fry. That’s the word on the streets."

Prison officials said that Williams refused his last meal at 6pm and the chance to meet a spiritual adviser. Instead, he spent his last moments drinking milk in a holding cell and reading letters sent to him from supporters. A handful of friends were admitted to see him for the last time.

"He’s complacent, quiet and thoughtful," said Terry Thornton, a Corrections Department spokeswoman.

In the minutes before his death, Williams was strapped to a trolley and wheeled into San Quentin's execution chamber. Witnesses said the nurse who delivered the lethal injection had trouble finding a vein in his muscular arms.

Steve Ornoski, the prison warden, said Williams offered to help. "He did seem frustrated," Ornoski said.

According to execution procedure, Williams would have been given a sedative, followed by potassium chloride, to paralyse him, then pancuronium bromide to induce a heart attack. He was the 12th person executed in California since the death penalty was reinstated in 1977.

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