The attack was reported around 3 p.m. Saturday near 18th and Bryant streets, a short distance away from the festival site.
The 48-year-old victim had a previous disagreement with the suspect and the two began arguing. The suspect then struck the victim with some sort of weapon, knocking him to the ground, where he began kicking him, according to police.
The suspect, described as a man around 50 years old, fled and had not been found as of this afternoon, police said.
The victim was taken to San Francisco General Hospital to be treated for a head laceration, an injury that is considered life-threatening, according to police.
Anyone with information about the case is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.