Thousands joined unplanned protests in Hong Kong after a territorial government banned a face mask that comes into effect at midnight (16:00 GMT).
Leader Kerry Lam referred to the extraordinary law of the colonial era, trying to quell the monthly anti-government riots.
The ban comes after the escalation of violence during protests on October 1
Protesters took to the streets immediately after the ban was announced.
Many left early to join spontaneous demonstrations. Some rabid protesters were blocking roads, setting fire to Chinese flags and vandalized stations and businesses when police fired tear gas.
Mass Transit Territory (MTR) has announced the complete cessation of operation of all trains and almost all bus routes. A spokeswoman told the BBC it was linked to vandalism and attacks on staff.
- Hong Kong protests are explained in 100 and 500 words.
Ms Lam stated that she had to apply the law of the colonial era because the protest violence that now "destroys the city weekly" and she could not allow the situation to "get worse and worse".
End of anonymity
Danny Vincent, BBC News, Hong Kong
Angry protesters chanted as they passed through the heart of this financial center They broke a banner celebrating 70 years of communist rule by setting it in front of a crowd.
Hundreds wore masks against the ban.
Today is the last time demonstrators can hide their faces legally. Anonymity has become a key part of this movement, but many fear that the introduction of this extraordinary law could lead to further restrictions.
This protest movement began in opposition to the extradition bill, which has now been removed from the legislature. The use of emergency law did not have to go through the legislature.
Kerry Lam insists that the territory is not in a state of emergency, but the law allows it to take further emergency measures.
What is the reaction to the ban?
Observers say that the regulation will be difficult to force and very controversial: critics warned that the ban on the mask may be the first in a series of "draconian" measures.
"This is a turning point. This is a Rubicon," pro-Democratic MP Claudia Mo told AFP. "And I'm worried that this may be just the beginning. More drastic bans in the law can hide it around the corner."
The announcement also sparked a reaction from a further position with Martha Hurtado, a UN man Spokesman for human rights, saying at a press conference in Geneva that "any restriction should be based on the law and be proportionate and at least in the # 39; scary. "
Meanwhile, UK Foreign Secretary Dominic Raab has stated that "political dialogue is the only way to resolve the situation".
Where is the prohibition applied?
The ban will extend to approved and disapproved public meetings – rallies and marches – as well as unlawful meetings and riots.
The prohibition covers all types of face, including face paint. Protesters are increasingly wearing masks for a variety of reasons, including hiding their identity – from employers, parents and, in some cases, police – and protecting themselves from tear gas.
There are exceptions for people wearing masks for health reasons or when required by the profession.
Ms Lam said that "the violence had escalated to alarming levels", leading to a situation of chaos and panic in the city.
What is the Emergency Ordinance?
The legislation referred to by Ms. Lam, called the Emergency Regulation, dates from 1922 and has not been in force for more than 50 years.
This allows the management to bypass the ordinary legislative process when bills must pass through the City Parliament, the Legislative Council.
The ERO was last used in 1967 to prevent unrest in the mall.
Ms Lam stressed that the new regulation did not mean that Hong Kong was in a state of emergency. But she said the city was "in grave public danger."
What is the background?
In June, protests began in Hong Kong over proposals to extradite suspected criminals to mainland China.
Since then, the extradition bill has been repealed, but protests have grown into democratic and anti-police demonstrations.
Over the months, clashes between police and activists have become more violent. On Tuesday, police first shot a protestor with a live bullet in the chest.
Authorities say an 18-year-old man, who attacked police and was arrested after the shooting, is in hospital in hospital.
Hong Kong is a former British colony returned to China in 1997.  There is a "one country, two systems" agreement that guarantees her some independence, and her people have certain freedoms, including freedom of assembly and freedom of speech.
But these freedoms – the Basic Law – will expire in 2047, and it is not clear what Hong Kong's status will be.