Officers had fired warning shots on multiple occasions in the past – two warning shots were also fired in Yau Ma Tei on Tuesday – but no one had been hurt.
Video footage of the shooting has been circulating widely online.
It shows the student who was shot was among a group of masked, black-shirted protesters who were huddled around a fallen officer, beating him with umbrellas and sticks on Tai Ha Road.
Another officer then approached the melee, with his pistol drawn and aimed at the protesters.
One of the protesters lashed out at the policeman with what appeared to be a metal rod. The officer kicked back, and quickly fired off one round at close range. No warning shot could be heard on the video.
The shot protester then staggered backwards several steps, and tripped backwards over the officer on the ground who the mob had been beating.
Another protester immediately ran to the fallen student in an apparent attempt to help, but was immediately subdued.
Other protesters then hurled a petrol bomb at the police, who then apparently withdrew.
The footage also shows the injured protester yelling out “My chest hurts a lot”, along with his full name and identity card number as volunteer first-aid workers attempted to help him as he lay bleeding on the pavement.
He is a secondary five student of a school in Tsuen Wan.
He was rushed to Princess Margaret Hospital, and then transferred to the Queen Elizabeth Hospital, where a source said he is to undergo surgery. He was in a critical condition on Tuesday night.
Meanwhile, live rounds were heard in Yau Ma Tei after protesters surrounded a group of officers and began beating them.
TV footage showed at least two officers with blood pouring down their faces as they retreated into a van. A Molotov cocktail was then thrown at the vehicle, prompting officers to get back out to tackle the group again.
An officer was beaten after falling down and two shots then rang out.
Riot police were seen pointing their guns at protesters at close range and the crowd then fled towards Nathan Road.
Two masked men in plain clothes were seen joining the police in their van before it also left the scene.
The Hospital Authority said that by around 5.30pm, 15 injured people had been taken to hospital.
Last updated: 2019-10-01 HKT 22:27