Hong Kong's chief executive Carrie Lam
has used a colonial-era emergency law to ban face masks after months of angry
The measure was announced on Friday, after the city saw widespread violence on 1 October, the 70th anniversary of Communist Party rule in China.
Ms Lam said the violence was "destroying the city" and authorities could not "just leave the situation to get worse and worse."
The ban comes into effect on Saturday.
The legislation invoked by Ms Law, called the Emergency Regulations Ordinance, dates to 1922 has not been used since there were riots in Hong Kong in 1967.
Hong Kong's protests started in June, sparked by proposals to extradite suspected criminals to mainland China.
The extradition bill has since been cancelled but protests have widened into broad pro-democracy demonstrations.
Over the months, clashes between police and activists have been becoming increasingly violent. On Tuesday, police shot a protester with a live bullet for the first time.