Images show several other officers also pointed guns at protesters who were chasing them with sticks and poles.
In another first, police deployed water cannon against protesters earlier in the day.
The protests began in the city's Tsuen Wan district and have since spread to the district of Tsim Sha Tsui.
Demonstrations were sparked by an extradition bill but have since morphed into broader anti-government protests, and developments on Sunday mark a serious escalation in the unrest.
