High school students gathered in the heart of Hong Kong's commercial district, chanting slogans and wearing posters reading "all people masked" on their uniforms. (AFP)

A ban on protesters wearing face masks took effect Saturday in Hong Kong under colonial-era emergency powers not used in 52 years, a move aimed at quelling months of unrest but which instead sparked rallies, widespread clashes and vows of defiance.

Chief executive Carrie Lam said she made the order under the Emergency Regulations Ordinances, a sweeping provision that allows her to bypass the legislature and make any law during a time of emergency or public danger.

“We believe that the new law will create a deterrent effect against masked violent protesters and rioters, and will assist the police” in law enforcement, Lam said on Friday.

Widespread protests immediately broke out across Hong Kong. Large crowds of mostly office workers blocked roads in the heart of the commercial district.

Some protesters tore down pro-China banners before clashes erupted throughout the evening. Police used tear gas in multiple locations to disperse protesters who had taken over roads, vandalised subway stations, set street fires and trashed pro-China businesses.

In the northern district of Yuen Long, a police officer opened fire when he was surrounded in his car and attacked by protesters, a petrol bomb exploding at his feet.

“A large crowd of rioters attacked a plainclothes officer. He fell into the ground, then got beaten by a lot of people. Under this life-threatening situation, the police officer fired a live round for his safety,” and lost the magazine of his weapon during the incident, police said in a statement.

Also in Yuen Long, a 14-year-old boy was shot and wounded by a live round, the South China Morning Post reported, citing a medical source. It was unclear if that round was linked to the plainclothes officer who opened fire.

The entire subway network was suspended, leaving protesters, locals and Friday night revellers stranded.

“Youngsters are risking their lives. They don’t mind being jailed for 10 years, so wearing masks is not a problem,” a 34-year-old office worker wearing a surgical mask, who gave her first name as Mary, told AFP.

Beijing described the ban as “extremely necessary.”

“The current chaos in Hong Kong cannot continue indefinitely,” Yang Guang, spokesman for the Hong Kong and Macao Affairs Office of China’s central government, said in a statement.

“An important moment has come for stopping the violence with a clearer attitude and more effective measures,” he added.

Critics said Lam’s move was a major step towards authoritarianism for Hong Kong, which has been governed by China under a “one country, two systems” framework since British colonial rule ended in 1997.

“This is a watershed. This is a Rubicon,” pro-democracy lawmaker Claudia Mo told AFP.

“And I’m worried this could be just a starter. More draconian bans in the name of law could be lurking around the corner.”

Prominent democracy activist Joshua Wong said the law “marks the beginning of the end of Hong Kong.”

“It is ironic that a colonial-era weapon is being used by the Hong Kong government and the Chinese Communist Party,” he told AFP.