One of the injuries to an elderly man who fell and hit his head was considered life-threatening, San Francisco fire spokeswoman Mindy Talmadge told The Associated Press.
Four other people were taken to a hospital with non-life-threatening injuries.
It appeared a 1.5-inch bolt stuck in the cable car's track caused the sudden stop, San Francisco Municipal Transportation Agency Director John Haley told reporters at the scene.
Officials believe the bolt doesn't belong to other cable cars that travel the route. Two other cable cars that passed the site about 10 minutes before the accident were being inspected.
San Francisco Police Officer Mary Godfrey told KTVU-TV a female passenger suffered a leg injury when she was thrown from the cable car.
Police told the station that the cable car grip man bit his tongue and the brakeman injured his ribs.