Officers responded to a report of a robbery at Al's Liquor at 479 Grand Ave. at around 5:40 p.m.
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Two men entered the store and began taking numerous bottles of liquor, police said. When a concerned citizen tried to intervene, the suspects smashed a bottle on the back of his head, causing a serious laceration.
The victim was taken to a nearby hospital to be treated for the injury, which was not considered life-threatening.
The suspects fled with two other males in a green four-door Mercury Marquis. They were described as 18- to 20-year-old white men, one around 6 feet tall, the other around 5 feet 11 inches tall with a moustache.
Anyone with information about the robbery and assault is asked to contact South San Francisco police Officer Saunders at (650) 877-8900, or Sgt. Campbell at (650) 877-8910.