Lam, speaking at a news conference, said a ban on face masks would take effect on Saturday under the emergency laws that allow authorities to "make any regulations whatsoever" in whatever they deem to be in the public interest.
"Civil Human Rights Front believes that even though the police and the Appeal Board on Public Meetings and Processions will continue to oppose to our efforts in organizing lawful protests, Hong Kong people will not be fearful and back down easily", it said.
Police have said the charges against Tsang would be filed on Thursday afternoon.
"Liberate Hong Kong, Revolution of our times" they shouted.
Local media reported that Hong Kong's Chief Executive Carrie Lam will hold a special Executive Council meeting Friday to discuss a ban on the masks, which have helped protesters hide their identities, and other tough measures under colonial-era emergency law. "By deleting this line, it gives the impression that the police force will support officers in whatever they do", she said.
A pro-government group, including lawmakers and lawyers, said Thursday that authorities should use the example of a Canadian law that imposes a jail sentence of up to 10 years on anyone wearing a mask during a riot or unlawful assembly.
Protesters took to the street chanting, 'Wearing masks is not a crime!'
Activists and many legislators have warned the mask ban could be counterproductive, impractical and hard to enforce in a city bubbling with anger and where tens of thousands have often defied police bans on rallies.
While police in the former British colony have always been admired for their professionalism compared with some forces elsewhere in Asia, the public has become increasingly hostile toward the police in recent weeks, amid accusations of heavy-handed tactics.
Protesters, some wearing gas masks and helmets, marched past some of the city's most expensive real estate including British bank HSBC's head office, on Friday, calling out for "five demands, not one less".
The government probably hopes that by banning face masks, it would be able to plant the fear of identification, and future retribution by the state, in the minds of protesters, which would act as a deterrence.
The shooting occurred during widespread violence across the semi-autonomous Chinese territory that marred China's National Day celebrations and has deepened anger against police, who have been accused of being heavy-handed against protesters.
"After so many months the government has refused to answer our demands", said one protester, who asked to be identified as just Chan, at a demonstration in the city's Central district. "The government is trying to intimidate us but at this moment, I don't think the people will be scared".
Lam added that while the emergency ordinance is being enacted to ban the masks, Hong Kong itself is not in a state of emergency. Lam said she would go to the legislature later to get legal backing for the rule.
The teenager who was the first victim of police gunfire in Hong Kong's months-long pro-democracy protests is to be charged with attacking police and rioting.
That period saw terror attacks by leftist militia's against police officers, the public and members of the media, leading to 51 deaths on both sides by the end of the year.
One activist was shot in the chest by police on Tuesday, the first injury from live ammunition since the protests began. At that point, Beijing can make any law for Hong Kong it deems fit to deal with the crisis.