"SFPD is on scene investigating a shooting that occurred. 3 shooting victims at this time. All are being transported to the hospital," the department tweeted.
Law enforcers have asked residents and tourists to avoid the popular hillside park overlooking the downtown while a police operation is under way to find the shooter.
San Francisco General Hospital spokesman Brent Andrew said one of the victims was in critical condition, according to the Chicago Tribune newspaper. Another one was released, he said.