Gov. Jerry Brown has repeatedly said that the state has gone as far as it can to release low-level offenders and reduce crowding at the prisons, and that it is providing adequate medical care for inmates. But last month, a federal judge criticized the system for allowing potentially lethal valley fever to spread through two jails in Central Valley and ordered the state to move 2,600 inmates at risk of catching the disease.

A small group of inmates in solitary confinement at the maximum-security Pelican Bay State Prison, in a remote area near the Oregon border, called for the protest months ago. They have complained that inmates are being held in isolation indefinitely for having ties to prison gangs. Some have been held for decades without phone calls, access to rehabilitation programs or time outdoors.

Ten inmates at High Desert State Prison in Northern California began their own hunger strike last week and were being monitored by medical staff for signs of distress, officials said. Their demands, made in a letter, include cleaner prison facilities, better food and more access to the prison library. Prisoners at several other facilities also issued demand letters, which were displayed on a Web site supporting the strikers.

The organizers timed the protest to coincide with the start of Ramadan, the Muslim month of fasting, which began this week; state officials said that would make it more complicated to determine how many prisoners were fasting out of religious obligation rather than in protest.

Prison officials said the protests had not caused any major disruptions.

“These actions have been talked about for months,” said Jeffrey Callison, a spokesman for the corrections department. “We have been preparing to make sure that the rules are enforced consistently.”

After the protests two years ago, corrections officials promised to use new criteria in placing inmates in solitary confinement and to create a process by which inmates could get out of isolation. Corrections officials say that of 382 inmates who have been screened, roughly half have qualified to return to the general population. But about 10,000 inmates remain in solitary confinement units.

Carol Strickman, a lawyer with Legal Services for Prisoners With Children who negotiated on behalf of inmates during the last hunger strike, said allies of the inmates had no way of verifying how many were taking part this time. During the last strike, officials prohibited participants from communicating with family and friends.

“Officials have this bunker mentality, but now it’s like a house of cards is falling down,” Ms. Strickman said. “There have been so many problems for decades, and now they are being forced to deal with them all at once.”