Some 30,000 inmates held in California state prisons refused meals on Monday as part of the start of what could be the largest prison protest in state history, the Los Angeles Times reported. It is the third hunger strike and work stoppage prisoners have waged in recent years as they protest the treatment of those held in indefinite isolation.

The planned protest, announced back in January, is a response to inaction on the part of the California Department of Corrections to follow up on promised reforms, advocates say. At the heart of the protest is the state prison system’s treatment of inmates who are labeled prison gang members, and then locked up in solitary confinement. They are kept there for over 22 hours a day in small cell where they’re forbidden physical contact and social interaction. 

The United Nations has found that just 15 days in solitary confinement violates human rights standards and can do irreperable psychological harm to a person, according to a lawsuit filed by the Center for Constitutional Rights. Yet hundreds of California inmates have been in indefinite isolation for more than a decade, according to Amnesty International. 

At its height in 2011, between 6,000 and 11,600 inmates participated in a hunger strike, the Los Angeles Times reported. The disparity is in part because the state records nine consecutive declined meals as a hunger strike and participation fluctuated. That 2011 strike resulted in policy changes and promises to revise policies. But two years on, "rather than improving, conditions have actually significantly deteriorated," said Amnesty International‘s Angela Wright.