The crash was reported shortly before 5 p.m. on Interstate 215 in Perris. All northbound lanes were shut as firefighters and emergency responders rushed to the scene, Riverside County Fire Department Capt. Rick Griggs said from the scene.
Six passengers who were trapped in the back of the bus had to be extricated by rescuers, he said.
Besides the eight who were critically hurt, the other 20 suffered moderate injuries, he said. All were taken to nearby hospitals for treatment.
The cause of the crash was under investigation. Griggs said he had no immediate information on the bus' origin or destination.
A call to a California Highway Patrol spokesman was not immediately returned.
Perris is about 60 miles southeast of Los Angeles.