The incident occurred on Tuesday, as China marked its 70th founding anniversary with a massive parade in Beijing.
The protesters in strife-torn Hong Kong called their rallies the “Day of Grief” protests, in an attempted affront to China, Presstv Reported.
The demonstrators ignored a ban on rallies and took to the streets across the city, with running clashes raging for hours across multiple locations.
According to witnesses, protesters threw rocks and petrol bombs at riot police, who responded for the most part with tear gas canisters and by shooting rubber bullets and water cannon in an attempt to break up the crowds.
The shooting incident occurred in Tsuen Wan district, where a police officer fired his weapon at a young man at close range after his unit was assaulted by demonstrators armed with poles.
Police said the officer feared for his life when he unloaded the weapon, hitting an 18-year-old man, who received first aid from officers before paramedics came to the scene and took him to a nearby hospital.
According to medical authorities, 31 people were admitted to hospital, two in a critical condition.
Since June, Hong Kong has been beset by unrest over a contentious extradition bill that would have allowed the extradition of suspects to mainland courts for trial. The bill has since been fully withdrawn, but the protests have continued.
The government of the semi-autonomous Chinese region has also opened talks with protesters.
Hong Kong has been governed under a “one-country, two-system” model since the city — a former British colony — was returned to China in 1997.
China has accused foreign governments, including the United States and Britain in particular, of provoking unrest in Hong Kong. The two countries have been voicing support for the protesters.