Thousands of defiant masked protesters streamed into Hong Kong streets after the city's embattled leader invoked rarely used emergency powers to ban masks at rallies.

Hong Kong's leader, Carrie Lam, said at an afternoon news conference earlier on Friday that the mask ban "will be an effective deterrent to radical behaviour".

The emergency law has not been used since riots in 1967. ( AP: Vincent Thian )

The ban came after an uptick in violence this week, which saw a police officer shoot a teenage protester, marking the first time live ammunition has been used against a demonstrator in the months-long unrest.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 17 seconds 17 s The move comes after a protester was shot by police earlier this week.

However, protesters and their supporters say such a ban won't stop the protests, and some police officers have said it will be difficult to enforce.

Here's what you need to know.

What is the emergency law?

Protests have been planned specifically to protest against use of the emergency law and mask ban. ( ABC News: Mary Lloyd )

The Emergency Regulations Ordinance came into effect in 1922 when Hong Kong was still under British colonial rule, and was a response to strikes at the city's ports.

In times of emergency or public danger, it gives Hong Kong's Chief Executive wide-ranging powers to make, "any regulations whatsoever which he may consider desirable in the public interest".

The emergency law has not been used since 1967, during violent pro-Communist protests against the British, inspired by the Cultural Revolution in mainland China.

That period saw terror attacks by leftist militias against police officers, the public and members of the media, leading to 51 deaths on both sides by the end of the year.

The law allows for increased censorship, arrests and detentions, the searching of premises and a wide range of other actions — the wording is very general, and can be applied however the leader sees fit.

The punishment for breaking any regulation put in place under the law is determined by Hong Kong's leader, up to a maximum penalty of life imprisonment.

What are the implications of resurrecting the law?

Many protesters have worn widely available protective masks during demonstrations. ( AP: Gemunu Amarasinghe )

Basically, it means Hong Kong's current Chief Executive, Carrie Lam, is able to make any sort of regulation she thinks is necessary to address the unrest — including banning Hongkongers from wearing face masks to hide their identity.

Ms Lam is under pressure to end the protests which are becoming increasingly violent. ( AP: Jae C. Hong )

Pro-Beijing politician Elizabeth Quat told reporters yesterday, the ban would be along the lines of similar legislation in Canada, passed into law in 2013.

"We will refer to the Canadian anti-mask law, which is mainly implemented for illegal gatherings and violent acts," Ms Quat said.

"Exceptions can be made if someone can prove they have a 'lawful excuse' for covering their face, such as religious or medical reasons."

The Canadian law carries a maximum jail term of 10 years, and some pro-Beijing politicians are expected to argue in favour of a similar punishment in Hong Kong.

What's the response and what might happen next?

Some police officers have suggested the ban would be too difficult to enforce. ( AP: Vincent Yu )

Pro-Beijing politicians and some in the police force have suggested a mask ban would help reduce the numbers of protesters on the streets, but others think it would just make the situation more heated.

Dennis Kwok, a pro-democracy politician representing the legal sector, said the ban would not stop the protesters and warned it could lead to further emergency regulations down the track.

"They can use it to enact an anti-mask law now, the next thing is they are going to extend the detention period," he said.

Other politicians said protesters would simply defy the mask ban, just as they have defied police bans on public rallies over the past few months.

Martin Lee, a veteran Hong Kong Democrat, said the only reason an "ancient, colonial set of regulations" are reintroduced is because "you can't legislate anymore".

"Once you start, there's no end to it. She could even invoke Article 23 [which prohibit treason, sedition, subversion and secession] under this," Mr Lee said. "That gives her justification — I need more police powers, I need more draconian laws — it's so handy.

"She is completely destroying the rule of law. Only the government can really harm the rule of law, citizens cannot."

The South China Morning Post spoke with several anonymous police officers who said they were already overstretched responding to the unrest.

Protesters are planning rallies in opposition to the proposed mask ban this weekend. ( AP: Vincent Yu )

One officer told the paper it would take too much time for police to check if someone had a valid medical reason for wearing a mask.

ABC/Wires