Critics said the move was a major step towards authoritarianism for Hong Kong.

Hong Kong's leader invoked colonial-era emergency powers Friday to ban protesters wearing face masks, but the move aimed at quelling months of unrest sparked immediate fresh rallies and vows to defy the new law.

Chief executive Carrie Lam said she had made the order under the Emergency Regulations Ordinances, a sweeping provision that grants her the ability 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 its law enforcement," Lam said.

But as soon as the law was announced masked protesters built barricades in the heart of Hong Kong's commercial district and began holding flash-mob rallies in multiple district.

Online forums used by protesters also filled with anger and vows to hit the streets over the upcoming three-day weekend.

The largest impromptu rally on Friday broke out in Central, where many blue-chip international firms are based, as protesters used plastic barriers, wooden pallets and traffic cones to set up makeshift blockades.

Thousands more masked protesters, many of them office workers, remained on the roads behind the main barricade.

"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.

Critics said the 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.

new law threatens anyone wearing masks at legal and unsanctioned protests with up to one year in prison

The last time the law was invoked was during the 1967 riots -- a period where more than 50 people were killed in a year-long leftist bombing and murder spree.

Months of unrest

Hong Kong's protests were ignited by a now-scrapped plan to allow extraditions to the mainland, which fuelled fears of an erosion of liberties promised under "one country, two systems".

After Beijing and local leaders took a hardline, the protests snowballed into a wider movement calling for more democratic freedoms and police accountability.

Protesters have used face masks to avoid identification and respirators to protect themselves from tear gas.

The ban came after Hong Kong was rocked by the worst violence of the year on Tuesday, the same day China celebrated 70 years of Communist Party rule.

Street battles raged for hours between riot police and hardcore protesters.

A teenager who was part of a group that attacked police with umbrellas and poles was shot in the chest with a live round -- the first such shooting since the demonstrations began.

Tough Penalties

The new law, which Lam said would take effect at midnight, threatens anyone wearing masks at legal and unsanctioned protests with up to one year in prison.

People can still wear masks in the street, but must remove them if asked to by police.

Exemptions are available for religious and medical reasons and for those who need masks to do their jobs -- such as reporters.

Lam said she did not rule out further laws under the emergency provisions if the violence worsened.

The emergency laws allow the city's leader to make "any regulations whatsoever" in the event of an emergency or public danger without the need use parliament.

But Lam stressed Friday her use of the powers did not mean the government had officially declared a state of emergency.

Enforcement Doubts

It is not clear whether the face mask ban will be enforceable, with doubts over how to police potentially huge masses of people wearing masks.

Even moderate protesters have already shown a willingness to break the law in huge numbers, appearing at unsanctioned rallies in their tens of thousands.

Stop and searches by police frequently lead to angry crowds gathering to berate and throw projectiles at officers.

Simon Young, a law professor at the University of Hong Kong, said a mask ban might deter some moderates from hitting the streets.

"But it could well have the effect of bringing more people out simply because they feel the need to protest against the exercise of executive authority," he told AFP.