Tour bus in San Francisco careens through crowded Union Square, injuring 20, six critically

SAN FRANCISCO -- Twenty people have been injured, six critically, by a sightseeing bus that careened through crowded Union Square Friday afternoon, the city's fire chief said.

Chief Joanne Hayes-White says the most badly injured include a bicyclist who was mowed down by the speeding bus. The critically injured have been taken to San Francisco General Hospital.

The double-decker, open-top bus slammed into several cars and hit scaffolding at the site of an unfinished building in the packed square. The scaffolding collapsed onto the sidewalk, but it's not clear whether any pedestrians were hurt.

The fire chief says it's too early to determine whether driver error, a mechanical problem or other issues caused the accident.

The driver of the wayward tour bus that crashed into five vehicles is said to be in critical condition, KTVU-TV said. It is not known if the bus driver had a prior medical condition. Investigators said the driver will be tested for drugs and alcohol.

Calls and messages for the bus operator, City Sightseeing San Francisco, weren't immediately returned.