Rap star Snoop Dogg is set to attend a rally at California's San Quentin State Prison on Saturday to protest against the execution of a former gang leader.
Dogg was formerly associated with street gang the Crips
Stanley Tookie Williams was sentenced to death in 1981 for the murder of four people during robberies in Los Angeles.
Williams has subsequently earned several Nobel Peace Prize nominations for his teachings, urging young people to stay out of gangs.
Snoop Dogg's requests to visit Williams in prison have so far been denied.
"We were unable to provide him with a security clearance because of his arrest history," said San Quentin spokesman Vernell Crittendon.
The musician, real name Calvin Broadus, was once associated with the Crips, the notorious Los Angeles-area street gang that Williams co-founded.
The rapper has been arrested several times for weapons and drug-related offences, according to the California Department of Corrections and Rehabilitation.
Stanley Tookie Williams was a founder of the notorious Crips gang
In 1990 he was sentenced to four years in prison, but the sentence was suspended, said corrections spokeswoman Terry Thornton.
In 1993, Snoop Dogg faced charges in connection with a drive-by shooting, but was later cleared.
Earlier this week, the rap star appealed against the prison's decision to deny him visiting rights. The warden is due to make a fresh decision on Friday.
Rallies are taking place around the state in support of Williams, including the screening of a documentary film about Williams in San Francisco, hosted by actor Danny Glover.
An award-winning TV drama entitled Redemption: The Stan "Tookie" Williams Story, starring Jamie Foxx, has also drawn attention to his case.
Saturday's rally will be held outside San Quentin's gate, according to organisers Death Penalty Focus.
The execution is scheduled to take place on 13 December.