A witness said an elderly pedestrian was in the intersection of Post and Divisadero streets when he was struck by the tour bus at around 1 p.m.
The witness and others tried to get the bus driver to stop, but he may not have even known he'd hit the man.
The bus company's attorney says this is the first serious incident the company has had and said the bus registration and safety checks are current. They are working with police.
Frank Zarich witnessed the gruesome death of the elderly man who was trapped under the tour bus.
"Based on the light, the pedestrian was walking on a red light, I'm not sure if he entered the intersection on a green light, or not," Zarich said. "Then as the bus kept going I saw the guy stuck under the bus being dragged for a few feet and I'm yelling,'Stop, stop, stop.'"
Zarich says the man crossed Post street heading south on Divisadero while the bus, heading north, made a right hand turn onto Post Street.
It's unclear if the man was fully out of the intersection, or not. The driver was sitting on the curb side of the bus.
"At the point that he's banging on the window he's on the driver's side," Zarich said.
"It is early in the investigation to say who had the right away," said San Francisco police Lt. Wilfred Williams.
The driver of the San Francisco Deluxe Sightseeing Tour Bus stopped after the rear tires ran over the man. The company's attorney say the driver has been with Deluxe Sightseeing for around two years and his mandatory certifications are current.
"He looked kind of shocked. I don't think he knew what happened even," Zarich said.
"The tour bus company, as well as the driver, they are cooperating with our investigation at this time," Williams added.
Police say they've interviewed tour bus riders and witnesses. Detectives are searching for surveillance footage of the intersection. There is a camera on the nearby medical building.
This is the third tour bus crash in San Francisco in the last two months. Twenty people were hurt on Nov. 13 when a bus lost control at Union Square. CHP cited the company for mechanical problems and other issues in its fleet.
Then on Dec. 26, nine people were hurt when a tour bus crashed on the Embarcadero. Investigators said another driver cut off the bus.