A policeman has shot a protester during another day of violent demonstrations in Hong Kong.
Footage posted on Facebook showed the officer drawing his gun before grappling with a man at a roadblock.
When another man approached wearing a face mask, the officer fired at him, hitting him in the torso.
As the grapple continued, the officer fired another two rounds - although police said only one person was injured in total.
There have been a number of other flashpoints during the day, with police firing rubber bullets and tear gas in others parts of Hong Kong.
At one point, tear gas was fired in the central business district - a rare occurrence during working hours on a weekday.
What happened at the shooting?
The shooting occurred as protesters tried to block a junction at Sai Wan Ho on the north-east of the island. Police confirmed that one officer "discharged his service revolver" and that a man was shot.
After the shooting, footage showed the 21-year old protester lying with his eyes wide open and with blood around him.
He was in critical condition and underwent surgery, a Hospital Authority spokesman told the BBC.
The police said officers also drew firearms from their holsters in two other places.
But they denied what they called "totally false and malicious" reports that officers were ordered to "recklessly use their firearms" in Monday's operations.
It was the third time a police officer shot someone with live rounds since the Hong Kong protests began in June.
The first incident was during protests on 1 October when China was celebrating 70 years of communist rule. The second case was a teenage boy shot in the leg on 4 October.
What else has happened on Monday?
While most protests have taken place at weekends, protesters called for a general strike - that is, all workers not to work - on Monday.
In addition to the shooting, other incidents included:
At the Chinese University, police fired rubber bullets in response to protesters throwing bricks, At Hong Kong Polytechnic, police fired tear gas at a demonstration Police use tear gas to disperse protesters in the central business district Protesters used barricades to block roads in various places, leading to long traffic jams. MTR railway stations were also targeted Police say protesters dropped "large and heavy objects" on roads, endangering drivers In one video circulating online, a police motorcycle seems intentionally to ram into protesters.
Police described the activists as "radical protesters," conducting "extensive illegal acts" and urged them to stop any actions that threaten safety and obstruct the police.
Several universities have cancelled their classes for the day.
The latest violence comes after a student died in hospital last Friday after falling from a ledge in a car park, reportedly trying to flee from police tear gas.
Why are there protests in Hong Kong?
Hong Kong is part of China but as a former British colony it has some autonomy and people have more rights.
The protests started in June against plans to allow extradition to the mainland - which many feared would undermine the city's freedoms.
The bill was withdrawn in September but demonstrations continued and now call for full democracy and an inquiry into police behaviour.
Clashes between police and activists have become increasingly violent and in October the city banned all face masks.