Gloria Steinem and Bonnie Raitt also joined the hunger strike and sent to Gov. Jerry Brown Monday a letter, which calls the isolation units an "extension of the same inhumanity practiced at Abu Ghraib and Guantanamo Bay."
A demonstration at the Capitol was arranged for Tuesday by the National Religion Campaign Against Torture, the Los Angeles Times reported Monday.
Other notables who included their signatures on the letter to Brown included political critic Noam Chomsky, Buddhist scholar Robert Thurman and actor Peter Coyote, the Times said.
More than 630 inmates are participating in the hunger strike in nine California state prisons. The protest, which began July 8, is aimed at putting a five-year limit on how long inmates are held in isolation.