Police say a traffic officer who shot a protester in Sai Wan Ho on Monday morning "had no bad intentions" and he feared his gun was going to be taken from him.
The 21-year-old student shot in the abdomen was in a critical condition after undergoing surgery in Eastern Hospital.
At a press briefing, police said officers were in the Sai Wan Ho area to tackle "radical protesters" who had built barricades and started blocking roads from around 7am.
"Our officers arrived to disperse the mob and to clear the roads. One traffic officer, who was not in riot gear, arrested a person. Then several others surrounded him. The officer pulled out his service revolver as a warning. However, the protester did not stop attempting to snatch the revolver," said Chief Superintendent John Tse.
"At that time, the officer believed that it was very likely that the revolver would be snatched and the consequences would be disastrous. Death and casualties would have resulted," he added.
Tse said the officer fired three shots, including one that hit the young man.
"We certainly believe that the officer did not have bad intentions to hurt anyone. This case is under investigation by the Regional Crime Unit of Hong Kong Island. We will look at all CCTV videos and online videos to investigate the case in depth."
The force said the family of the officer involved in the shooting had since had his personal information leaked online and death threats had been made against his children.
"We appeal to everyone in society to stay calm and refrain from unlawful acts," Tse said.