Opponents of a face mask ban made two arguments, Mr. Tsang said. One was that it could be hard to enforce. France has such a ban, but it has not prevented many so-called yellow vest protesters from wearing them anyway.
Imposing a face mask ban could also hurt the government’s effort to persuade Hong Kong’s public, its tourists and the international business community that most of the city is safe most of the time, he said.
But while the government was deeply split on the issue through Tuesday, the escalation of violence on Tuesday night, including the first police shooting of a protester, left the authorities reconsidering every option, Mr. Tsang said.
“It appears we need more effective, more stringent measures,” he said.
As word of the possible ban spread on Thursday, masked student protesters said in interviews on Thursday afternoon that they would defy the ban.
They warned that a ban on masks could prompt more people to take to the streets and lead to more violence. Moderate protesters, who have tended to play a stabilizing role in crowds and tried to discourage attacks on individuals and businesses perceived as loyal to Beijing, might become less willing in the coming weeks to defy a ban on face masks, leaving the streets to more radical protesters, they said.
Mrs. Lam’s use of emergency powers allowed her to bypass the legislature, and underlines how her government and the police force may have run out of ways to restore order without limiting some civic freedoms. The measure alarmed some observers who feared she might exercise other emergency powers such as imposing curfews, muzzling the press, deporting foreigners and suspending various democratic rights.
“For the international community, any kind of emergency powers will send warning bells,” said Simon Young, a professor at the University of Hong Kong Law School. “Although it may start out with an incremental measure, then nothing’s to stop another measure from being added, and further measures from being added.”