“The crisis triggered by the extradition bill has completely changed in nature. Under foreign intervention, it has evolved into a Hong Kong version of a colour revolution,” the Hong Kong and Macau Affairs Office said in a statement read out during the prime-time evening news programme on state broadcaster CCTV.

Citing the “serious threat” to public safety posed by growing violence on the streets, the office said the Hong Kong government’s enactment of the anti-mask law was “legitimate, sensible, reasonable” and “extremely necessary”.

“The current chaos in Hong Kong cannot go on indefinitely. It is high time for us to take a more clear-cut attitude and more effective measures to immediately stop the violence and unrest,” it said.

It also underlined the central government’s “resolute support” for Lam, the Hong Kong government and police taking “all necessary measures” to punish all “violent criminals”, especially “the core members and the planners, organisers and commanders behind them”.

The mask ban will come into effect under the colonial-era Emergency Regulations Ordinance, invoked on Friday for the first time in more than half a century. The ordinance allows authorities to “make any regulations whatsoever” they deem to be in the public interest.

Opponents have called the move a slide towards authoritarianism and warned that it could further inflame tensions. On Friday night, thousands of protesters staged rallies across the city, blocking roads and vandalising subway stations in shows of anger and defiance.

But across the border in mainland China, the news was met with wide applause, from the government and state media to internet users.

The central government’s liaison office in Hong Kong also issued a statement of support for the ban and the Hong Kong government.

MTR stations and bank vandalised as protesters rage against Hong Kong leader Carrie Lam's anti-mask law

“The central government believes the chief executive and the special administrative region’s government can make good use of Hong Kong’s local and existing laws to stop the violence and restore order, and we offer our resolute support,” it said.

It also called on the Hong Kong public to support the government and the police in carrying out “all necessary measures” to maintain social stability.

In contrast to its silence on Tuesday’s protests, mainland media were quick to reporton Lam’s announcement of the mask ban.

In a commentary, Communist Party mouthpiece People’s Daily called the mask ban “an opportunity for Hong Kong”.

Hong Kong mask ban has precedent in France – but not Europe as a whole

But Qin Qianhong, a law professor and expert on Hong Kong’s Basic Law at Wuhan University, cautioned that the effect of the new law remained to be seen.

“It will be a test of the Hong Kong police of how well they can enforce the law,” Qin said, citing the challenge of effectively enforcing a broad ban.

However, Li Xiaobing, an expert on Beijing’s Hong Kong policy at Nankai University in Tianjin and a public advocate of using the emergency law, was more optimistic about its implementation.

“The ordinance is not only about the present, but more importantly about the future. It gives [the Hong Kong government] one more tool to rule Hong Kong. We don’t need to be overworried with its effectiveness in the near term,” he said.

On Friday, Lam declined to say if there was a sunset clause for the ban.

She said some past regulations made under the emergency law were repealed, but she could not say exactly when she would order it rescinded.