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.
Ms 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.