A policeman fired towards the sky in Tsuen Wan after his group of six officers were attacked by dozens of protesters in Tsuen Wan.
The six were outnumbered by protesters at the intersection of Chung On Street and Sha Tsui Road, being hit by metal rods and sticks.
Three officers pulled out their handguns and one shot was fired.
The confrontation came after dozens of protesters vandalized at least six shops on Yi Pei Square, including two mahjong parlors.
The square was where thugs attacked people with knives earlier this month.
Earlier in the evening, police deployed two water cannon vehicles to Tsuen Wan. Most protesters left Yeung Uk Street, a major site of confrontation, by 7pm.