Hong Kong’s chief executive Carrie Lam has used a colonial-era emergency law to ban face masks to try to quell months of anti-government unrest.
She said the ban would come into force on Saturday, after an escalation of violence during protests on 1 October.
Lam said the decision was taken because the situation could not be allowed “to get worse and worse”.
Protesters, who have promised to defy the new law, immediately took to the streets in response.
By 22:00 local time (14:00 GMT), a number of banks, buildings and stations had been vandalised by the demonstrators. Police had fired off multiple rounds of tear gas.
Mask-wearing activists had also earlier called on others to wear masks in defiance of the government, which critics fear is becoming increasingly authoritarian.
Lam said she was forced to implement the colonial-era law because the violence at the now-weekly protests was “destroying the city”.
Observers say the regulation will be hard to enforce and hugely controversial: critics have warned the mask ban could be the first in a series of “draconian” measures.
“This is a watershed. This is a Rubicon,” pro-democracy lawmaker Claudia Mo told news agency 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.”
The announcement has also prompted reaction from further afield, with Marta Hurtado, the United Nations human rights spokeswoman, noting at a press conference in Geneva that “any restriction must have a basis in law and be proportionate and as least intrusive as possible”.
UK Foreign Secretary Dominic Raab, meanwhile, has said “political dialogue is the only way to resolve the situation”.
But Beijing welcomed the move, saying the ban was “extremely necessary” to end the violent protests.