LOS ANGELES — Facing an unprecedented order from the Supreme Court to decrease its inmate population by 11,000 over the next three months and by 34,000 over the next two years, California prisons last week began to shift inmates to county jails and probation officers, starting what many believe will be a fundamental and far-reaching change in the nation’s largest corrections system.

Last spring, the Supreme Court ruled that overcrowding and poor conditions in state prisons violated inmates’ constitutional rights and, in a first, ordered a state to rapidly decrease its inmate population. Gov. Jerry Brown and the Legislature approved a plan that would place many more offenders in the custody of individual counties.

Under the plan, inmates who have committed nonviolent, nonserious and nonsexual offenses will be released back to the county probation system rather than to state parole officers. Those newly convicted of such crimes will be sent directly to the counties, which will decide if they should go to a local jail or to an alternative community program. And newly accused defendants may wear electronic monitoring bracelets while they await trial.

“This is the largest change in the California state system in my lifetime,” said Barry Krisberg, a law professor at the University of California, Berkeley, who has watched the state prisons for decades and testified in the Supreme Court case last year. “Given that what we had was completely broken and was the most expensive, overcrowded and least effective in America, there’s some hope that this will change it.”