A plainclothes police officer fired one shot after being sieged by a group of rioters in Hong Kong's Yuen Long district Friday night, authorities confirmed, saying the self-defense act was reasonable and legitimate.

The incident took place at around 9:00 p.m. when the officer fell onto the ground, then got beaten up and two petrol bombs thrown on him by the rioters, facing serious threats to his life, Hong Kong police said. 

Rioters also tried to grab the pistol of the officer while he was under attack.

The move was stopped by the officer, but bullets were missing at the scene, said Yu Hoi-Kwan, senior superintendent of Police Public Relations Branch in a midnight briefing, adding a follow-up investigation will be opened.

Except for the incident in Yuen Long, violent protests have spread to 14 districts in Hong Kong after Friday afternoon, the same day Hong Kong government announced it will invoke the "anti-mask law" in a bid to restore order in the city after months-long turmoil.

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Rioters committed destructive acts extensively on Friday including setting fire, vandalizing MTR facilities, smashing glass panes of shops, throwing bricks and petrol bombs, and beating up MTR staff and bystanders.

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Speaking to reporters, the police appealed for unity and zero tolerance towards any violence, calling those acts over the past months "wearing."

"We hope that all sectors of society will work together, and not condone any acts of violence, and restore social order," police said at the briefing.