Someone sprayed pepper spray for unknown reasons at about 6:20 p.m. at one of the Powell Street entrances to Muni, San Francisco Municipal Transportation Agency spokesman Paul Rose said.
No one was seriously injured, but two station agents and two bystanders were taken to a local hospital as a precaution. An additional two were sprayed but did not need to go to the hospital, Rose said.
Rose could not say if any arrests were made. A police spokesperson was not immediately available for comment.