A police officer shot and wounded a pipe-wielding protester in self-defense and to save his colleagues, a police spokesperson said on Tuesday.

Senior Superintendent of the Police Public Relations Branch Yu Hoi-Kwan said that the shooting took place when a mob attacked a small group of police officers in Tsuen Wan.

The protester, 18, was shot in his left collar bone and was conscious on the way to Princess Margaret Hospital, Yu said.

The police did not wish to hurt anyone and was saddened to see it happen, she added.

The spokesperson warned rioters to immediately stop committing illegal acts and urged members of the public to stay away from illegal assemblies.

In a video filmed by the University of Hong Kong Student Union's Campus TV, the 18-year-old was seen attacking the police officer with a steel pipe that hit the officer's revolver-holding arm and touched off the shot that wounded the man.

The wounded man, surnamed Tsang, is a form-five secondary student at the Tsuen Wan Public Ho Chuen Yiu Memorial College.

According to the Hospital Authority, by 7:30 pm, 31 people were sent to hospitals, including two in critical condition and one in serious condition.