"Stanley Tookie Williams is not just a regular old guy, he's an inspirator," the rapper and former Crips member told a crowd of about 1,000 outside San Quentin State Prison.
Williams, 51, is set to be executed Dec. 13. He was convicted in 1979 of murdering four people during two robberies in Los Angeles. He has exhausted his appeals and has asked Schwarzenegger for clemency.
Schwarzenegger said this past week that he was "dreading" the decision.
Williams founded the Crips with a childhood friend in 1971 in Los Angeles, and in the following years, the gang battled with its rival, the Bloods, for territory and control of the drug trade.
In prison, however, he gained international acclaim for writing children's books about the dangers of gang life. An award-winning television movie starring Jamie Foxx, "Redemption," was based on his life.
Snoop Dogg, 33, whose real name is Calvin Broadus, said Williams inspired him to work with young people. The rapper said he was once a gang member and now does youth outreach activities, including running a football league for youngsters.
"I didn't get this from somebody that was on the streets. I got this from Stanley Tookie Williams, a brother that was locked up on death row," he said, wearing a white T-shirt with huge black letters that said savetookie.org. "He inspired me to want to do something positive with my life and to go touch the kids."
The rapper had wanted to visit Williams on death row, but his application was denied by prison officials because of his criminal record. Snoop Dogg has been arrested several times for weapons and drug-related offenses, according to the California Department of Corrections and Rehabilitation.
Organizers played a new Snoop Dogg song called "Real Soon," which promotes Williams' advocacy work.
Todd Chretien, who works with the Campaign to End the Death Penalty, an advocacy group that helped organize the rally, implored the governor to grant clemency.
"There is no reason on earth to kill him, and there is every reason to keep him alive," he said.