Thousands of protesters are continuing to defy a ban on wearing masks in Hong Kong as clashes have again taken place between demonstrators and authorities.

A petrol bomb was thrown at the gate of a metro station, and two government offices and a cafe were vandalised, although the mood on Saturday was less tense than at recent protests because police had not used teargas.

Late on Saturday, however, the mood changed when police fired a shot outside the Mong Kok police station. The shot, possibly a rubber bullet or sponge round, was fired after a small group of protesters shouted obscenities and shone laser at police officers, who rushed out to subdue and beat a couple of young men.

Earlier in the day, crowds of people mostly dressed in black gathered at the seafront of the tourist area of Tsim Sha Tsui on Saturday afternoon before marching against the government’s invocation of the colonial-era emergency regulations ordinance just over a week ago.

The application of the law bypassed the legislature to forbid the use of facemasks at protests and public assemblies in an attempt to quell anti-government protests that are in their 18th week.

Since the ordinance was implemented, however, it has provoked several more violent demonstrations as people who feared the government would arbitrarily introduce more draconian measures to suppress civil liberties.

Many protesters wore medical masks on Saturday but a few also wore balaclavas. A large number of riot police were deployed in different districts along the route of the march, with protesters taking over the main thoroughfare, bringing traffic to a standstill.

Onlookers jeered and yelled obscenities at officers who wore black face masks and obscured their police numbers.

The atmosphere became tense when an officer rushed up to a young couple who wore medical masks but were not part of the protest. “Take off your bloody masks. Show your identity cards,” he shouted. Another officer said: “See a doctor if you’re ill.”

Passersby shouted back at police. One young father, holding his infant daughter, told them: “Beware of what sort of a world you’re creating for your own children.” Several police officers rushed up to intimidate him.

His wife said through tears: “We’re protesting today precisely because we have a child. If we don’t speak up for others now, who will speak up for us?”

Outside Prince Edward metro station, people lit incense and paid their respects at a makeshift shrine decorated with white flowers. Many believed people died at the scene after being beaten by police in late August, and have accused the government of a cover-up.

A 55-year-old man, who burst into tears after bowing in front of the shrine, said: “I’m convinced that people have died here – I have no trust in the government.”

A police officer shouted at and pointed a metal stick at the face of an elderly woman who was trying to mediate between him and an angry protester, leaving her trembling.

Meanwhile, marchers vandalised two government offices and a coffee shop seen as being pro-Beijing, and a petrol bomb was thrown at the gate of a metro station not on the route of the protest. No one was hurt in the incident.

Protesters smashed glass and sprayed graffiti on the walls of the Kowloon and Cheung Sha Wan government offices. Police said the demonstrators also entered the Cheung Sha Wan offices and started a fire.

Elsewhere, hundreds of masked protesters chanted slogans such as “I have every right to wear a mask” and sang the protest anthem Glory to Hong Kong at a shopping centre in Sha Tin. Some had their faces painted while others played musical instruments.

A 15-year-old student said: “I can’t refrain from wearing a mask just because of fear. We should resist an unjust law and an unjust regime.”

Dozens of older people staged a sit-in protest outside the police headquarters to protest against the beating and arrests of people as young as 12. Hong Kong officials said one-third of the more than 2,200 protesters arrested during the four months of unrest were under 18.

Hong Kong police issued a statement condemning vandalism and unauthorised assembly.