Flash mobs and “wildcat protests” erupted across Hong Kong on Friday after Chief Executive Carrie Lam invoked colonial-era emergency powers and announced a ban on masks.

Lam said the ban on masks would go into effect on Saturday, with an exception for those who have a “legitimate need” to wear them.

Given that surgical masks of the sort favored by germ-conscious people are the most popular choice for protesters, Lam should expect a great many of them to insist they have such legitimate needs. Hong Kong residents were prone to wearing surgical masks in public even before the protests began.

“We must stop the violence,” said Lam, touting the mask ban as “an effective deterrent to radical behavior” because the anonymity provided by masks emboldens “unbridled” demonstrators to become violent.

Mask violations could be punished with up to a year in jail and a fine of $25,000 in Hong Kong dollars. Lam invoked an emergency law dating from British colonial days that has not been used since 1967. Critics said she was going too far and illegally bypassing the legislature, not to mention taking steps more likely to inflame public anger than induce calm.

“This is an ancient, colonial set of regulations, and you don’t use them unless you can’t legislate anymore. Once you start, there’s no end to it,” said activist lawyer Martin Lee.

Response to Lam’s announcement was immediately and strongly negative. Hundreds of people poured onto the streets in “flash mob” protests, many of them apparently business people who left their offices to demonstrate. Others were students still wearing their school uniforms.

LATEST: Protesters wearing masks fill the main roads of Hong Kong after the government announced a face mask ban pic.twitter.com/i2a27jnMR5 — Bloomberg TicToc (@tictoc) October 4, 2019

March in Central https://t.co/rxkJIYHsJD — Joshua Wong 黃之鋒 (@joshuawongcf) October 4, 2019

Many of the demonstrators wore masks, including the gas masks that discomfit the authorities because they shield against tear gas and pepper spray, and chanted “wearing a mask is not a crime.”

“The anti-mask law has become a tool of tyranny,” a student protester told Reuters. “They can make use of the emergency law to enact any policies or laws that the government wants. There’s no rule of law anymore. We can only be united and protest.”

Hong Kong businesses and schools announced they would close early on Friday, anticipating much larger demonstrations in the evening. Tear gas was reportedly fired in several locations.

The British government urged restraint on Friday, with Foreign Secretary Dominic Raab implying Lam made a grave error by invoking emergency powers.

“While governments need to ensure the security and safety of their people, they must avoid aggravating and instead reduce tensions,” said Raab.