Hong Kong police have urged the public to stay indoors after an 18-year-old protester was shot in the chest by an anti-riot officer during a clash.

Another policeman has been filmed shouting at random onlookers, threatening to throw Molotov cocktails at 'rioter' protesters as the city is rocked by unprecedented level of violence amid China's National Day celebrations. 

The injured demonstrator was hit in the chest by a police officer in the city's Tsuen Wan district this afternoon and was taken to Princess Margaret Hospital.

Debris from the bullet punctured one of his lungs and was lodged in his chest only an inch from his heart and he was in critical condition, according to reports. He is said to be a secondary school pupil.

According to latest reports, surgeons have removed the main part of the bullet from the teenager. His heart, liver and spine were not affected by the shot. 

Footage posted by The Hong Kong University Students' Union shows a police officer drawing the gun and shooting a male activist at close range as the protester beat the officer with a baton. 

UK has called the use of live ammunition in Hong Kong 'disproportionate'.