Peter Schey was joined by relatives of some of the inmates he represents at a news conference Tuesday, the Los Angeles Times reported.
"We have California treating several thousand prisoners in much the same way the U.S. government treats enemy combatants held in Guantanamo," Schey said.
California is holding about 4,000 inmates in segregation, including more than 1,000 at the notorious Pelican Bay prison. Most are suspected gang members or inmates who have committed violent crimes in prison.
Walter J. Coto, who is being held in segregation at Corcoran State Prison, was one of the inmates involved in the petition, writing, "Every single moment I've spent in these torture chambers has chipped away my humanity."
Jeffrey Callison, a spokesman for the California Department of Corrections and Rehabilitation, defended segregation as necessary to protect the rest of the prison population, but said the department is considering some changes. These are expected to include requiring more evidence of gang ties before putting someone in solitary and shortening stays there.
Almost 300 of the inmates at Pelican Bay have been in the prison for more than 10 years. The current policy is to segregate suspected gang members for at least six years unless they agree to quit the gang and provide information on it.