The Metro Blue Line train was headed to Long Beach carrying passengers while the bus was out of service. As many as two dozen people were hurt, but none of the injuries appeared to be life-threatening, Fire Department spokesman Brian Humphrey said.
"We had an out-of-service bus turn in front of the train," said Marc Littman, spokesman for the Metropolitan Transportation Authority, which operates both the train and the bus. "We don't know who had the right of way."
The impact knocked the front car of the electric train off the track. The other cars remained on the track. Littman said they were investigating the incident.
The crash comes just a week after a Metrolink commuter train smashed into a freight train in the San Fernando Valley, about 30 miles northwest of downtown, killing 25 people and injuring more than 130.
The Blue Line runs on 22 miles of track from downtown to Long Beach, with other routes through Los Angeles County.
Associated Press writers Noaki Schwartz and Jeff Wilson contributed to this report.