A 21-year-old man is clinging to life after being punched and suffering a head injury in downtown San Francisco on Tuesday, police said.
The man fell back and hit his head during the assault just off Market Street at the corner of Mason and Turk streets at around 6:30 p.m., according to police.
Police have not released a detailed description of his attacker, a man who has not been arrested.
The victim was taken to a local hospital with injuries considered life threatening.
It is not clear what prompted the attack.