A University of California, Davis bus carrying students and health workers overturned on a busy freeway interchange Thursday, injuring at least 20 people and snarling rush-hour traffic around the state capital.

The bus crashed shortly before 9 a.m., California Highway Patrol Sgt. Evan Williams said. All five lanes of traffic were closed on the freeway, part of the downtown business portion of Interstate 80.

"I woke up and people were screaming, and that's when it scraped against the dividers ... It was sliding on the ground for a while," an unidentified passenger told a reporter for KOVR-TV.

The bus was carrying about 40 passengers, mostly students and people who work at the health center, said Meagan Herda, a 20-year-old student assistant in pediatrics at the health center.

"I was doing homework at the time. All of a sudden we hit the railing and flipped. It seemed like maybe she (the bus driver) was overcompensating and trying to get out of the way of something," Herda said. "All of a sudden I was airborne. There were people and drinks everywhere."

A spokeswoman for the UC Davis Health Services division did not immediately return a telephone call seeking comment.