The crash was reported at about 7:45 a.m. near Masonic Avenue and Haight Street and involved a bus on the 37-Corbett line, spokesman Paul Rose said.
The driver of the bus was getting ready to start the outbound route, which begins at that intersection. When the driver took off the wheel block for the bus, it rolled backward and struck a vehicle, Rose said.
No passengers were on the bus at the time, and no injuries were reported, he said.
The driver of the bus has been placed on non-driving status and will be tested for drugs and alcohol as part of the agency's standard procedure for investigating an accident, Rose said.