Authorities in Los Angeles are concerned about possible rioting if the co-founder of the Crips street gang, Stanley Tookie Williams, is executed as planned.

Williams, 51, is scheduled to die by lethal injection at San Quentin State Prison on Tuesday.

However, California Gov. Arnold Schwarzenegger is currently weighing Williams' request for clemency. It's not clear when a decision on that might come.

Fearing a repeat of the 1992 race riots in which 52 people died, police, schools and community groups have been told to prepare for violence if clemency is not granted.

Robin Toma, executive director of the Los Angeles County Human Relations Commission, said the organization had received "credible" threats of violence if Williams is put to death.

There are also fears that Williams' execution could cause unrest in the prison system.

For that reason, all prisoners at San Quentin will be locked down during the execution, and there is the expectation that other state prisons will choose to do the same.

Williams has spent 24 years on death row for the shooting deaths of four people in 1979. He was convicted in 1981 of killing a convenience store worker and, days later, killing two motel owners and their daughter during a robbery.

His case has gained wide media attention because of the growing support of many -- including celebrities such as Oscar-winning actor Jamie Foxx and Archbishop Desmond Tutu -- who say Williams has turned his life around in jail.

Williams has written nine anti-gang books aimed at young people, been nominated several times for the Nobel Peace and Literature Prize, and his "Protocol for Street Peace" has been used by rival gangs to broker gang truces.

Prosecutors, and some of the families of Williams' victims, say nothing he does now changes the fact that Williams fatally shot four people.

The Crips co-founder denies committing the murders.

A California governor has not granted clemency since 1967, when Ronald Reagan spared the life of a brain-damaged killer.