Thousands of people swept into the streets of Hong Kong for a night of protests marked by violence after the government activated sweeping colonial-era powers for the first time in over half a century, using them to ban face masks.

The chief executive, Carrie Lam, also said harsher measures could be on the table if the protest movement continued, amid calls from police groups and pro-Beijing politicians for a citywide curfew, and discussion of delays to local elections set for November.

The government appeared braced for more clashes between police and protesters before Lam’s announcement that she had invoked the emergency regulations ordinance and passed a law forbidding the use of face masks.

Government employees were sent home early, all after-school activities were cancelled, and many shopping malls, banks and businesses shuttered.

Shortly after she finished a two-hour press conference, the first groups of protesters were filtering on to the streets of Hong Kong island.

After darkness fell, crowds set fire to two metro stations and vandalised shops and businesses considered pro-China; police responded with teargas, and in at least one case live ammunition.

Facebook Twitter Pinterest Riot police in Hong Kong on Friday. Photograph: Vincent Thian/AP

There were reports that a student had been hit in the thigh by a bullet, after a group of protesters attacked an off-duty police officer. Hospital authorities said they had received a 14-year-old boy, who was in a serious condition.

At the press conference to announce the new anti-mask law, Lam said that if violence was not halted the government would not rule out any measures permitted by law. “Freedoms are not without limits,” she told journalists.

The emergency regulations ordinance, created by British authorities to break up port strikes in 1922, had not been used for more than half a century, and never since the handover of Hong Kong to Chinese rule in 1997.

It gives Lam virtually unlimited powers, although legal scholars said the government could be challenged in court if anything it did violated the Basic Law, the territory’s constitution.

“We have the duty to use all available means to stop the escalating violence and restore calmness in society,” Lam said in a combative press conference unveiling the new law. “The decision to invoke the emergency regulations ordinance is a difficult, but also a necessary one for public interest.”

Masks have become common among protesters who fear either arrest now, or retaliation from Hong Kong and Chinese authorities in the longer term if they are identified. But the ban seemed unlikely to deter crowds who have already defied multiple government bans to turn out on illegal marches and demonstrations.

Facebook Twitter Pinterest Carrie Lam: ‘We would not rule out any measures that we can reasonably implement under the current law.’ Photograph: Vincent Thian/AP

Activists and rights groups warned the mask ban might pave the way for fiercer controls on public life, once the government had tested public and legal response to using the emergency ordinance.

Asked whether she would consider other measures proposed by pro-Beijing politicians, including a curfew or a delay to upcoming local elections, Lam said everything was on the table.

“If the violence escalates, as a responsible government, we must make use of existing legal provisions,” she said. “We would not rule out any measures that we can reasonably implement under the current law.”

Later she added: “Freedoms are not without limits, OK, so there are things which one has to strike a balance, dealing with a particular situation.”

Protesters, rights groups and academics denounced the use of emergency powers as a gross abuse of power, and a dangerous precedent for a city that for the last 20 years has enjoyed an unusual combination of civic freedoms without full democratic rule.

Play Video Face mask ban provokes fresh protests in Hong Kong – video

“Today, she declared that the authoritarian government can take away your freedoms any time,” said Prof Ma Ngok, political scientist at the Chinese University of Hong Kong. He added that the new law would likely prove impossible to enforce and counterproductive, provoking “an even larger and more violent resistance”.

The regulation came into effect at midnight on Friday and bars all face coverings, including paint, at any public gathering, on pain of a fine and a year in jail.

In the testy press conference, one journalist put on his gas mask and protective goggles and shouted at Lam: “Would I be arrested if I am dressed like this while working?” Officials said there would be exemptions for anyone who covered their face for health, religious or professional reasons.

Despite the sense of crisis, and the deployment of emergency powers, Lam insisted that the city was not in a state of emergency. The ordinance also covered situations where there was a “danger to public order”, she said.

The city was wracked by the worst violence of the four-month-old protest movement on Tuesday, as crowds fought pitched battles with police across the city and an officer shot a teenage student in the chest.

The government’s response to that chaos and and an outpouring of grief and anger over the injured student, suggested a worrying lack of flexibility in tackling the city’s crisis, said Jean-Philippe Béja, research emeritus professor at the centre for international studies and research at Sciences Po in Paris.

“The only thing that they have come up with is using a police response to solve political problems,” he said. “This is a further restriction on HK’s freedoms, but the way it is implemented is more worrying than the law itself because it’s completely arbitrary.”

• Additional reporting by Verna Yu

