When police moved to disperse the protesters, local residents objected, and officers found themselves facing angry crowds, some of whom turned violent.

Hundreds then blocked police vans from leaving, claiming arrested protesters were being taken away, and attacked the outnumbered officers, using fire extinguishers, helmets, umbrellas and other objects. Police used pepper spray and waved their batons at protesters and residents alike.

Tear gas was fired at the height of the chaos, prompting many to complain that, unlike the protesters, local residents had no protection against the fumes.

Late into the night, hardcore protesters in Wong Tai Sin targeted the disciplined services quarters, where off-duty officers and their families lived, with reports of explosives being let off outside the building and objects thrown, breaking windows on the lower floors.

Protesters with gas masks, helmets and makeshift weapons, shields and body armour clashed or faced off with fully geared riot police at multiple flare-up points, including Yau Ma Tei, late into the night, while others massed at Wong Tai Sin to vandalise the entrance of the local police station.

The police presence looked light during the afternoon as protesters choked the main artery of Nathan Road and its side streets from Mong Kok to Tsim Sha Tsui, and grew more emboldened with their actions as the evening wore on.

Shops close early, residents stay put during protest in Mong Kok

Police sprang a surprise at around 9.30pm, emerging in full riot gear with shields and batons from both stations to move in on the protesters swiftly after putting up a black final-warning flag.

They fired multiple rounds of tear gas and arrested groups of them, many of whom fought back.

After the first charge of the Special Tactical Squad, also known as the Raptors, protesters threw petrol bombs that landed some distance away from the police line.

Earlier, throughout the afternoon, protesters roamed the streets of the usually bustling retail hub with police keeping a mostly hands-off approach, and blocked the Cross-Harbour Tunnel temporarily with water-filled barricades they had emptied, as Hong Kong lurched into its third month of anti-government demonstrations with no resolution in sight.

Police fire tear gas in Tsim Sha Tsui, Mong Kok as protesters fling petrol bombs

The slogans they chanted went beyond the extradition bill, the dominant one being: “Liberate Hong Kong, revolution of our times.”

Many in the crowds also called for others to join citywide strikes on Monday, chanting, “Monday! Go on strike!”

“To take Nathan Road is a gesture to show to the government how many people actually came out today – it’s people’s power,” protester Martin Pang said.

A large group besieged Tsim Sha Tsui Police Station, defacing the walls with graffiti and starting a fire outside its gates. They also used slings to lob bricks over the wall at parked cars in the station premises and pointed laser beams at the officers inside, all the while shouting expletives.

Police closed their reporting room services – where people seeking the force’s help in person are received – at the Mong Kok, Tsim Sha Tsui, Hung Hom and Yau Ma Tei stations.

What is LIHKG and how did it become go-to forum for Hong Kong’s protesters?

On the ground, though, officers were barely visible at first when protesters began gathering in the playground in Mong Kok for their rally.

Organisers said 120,000 people took part; police estimated there were only 4,200 participants.

What began as a peaceful assembly, however, soon descended into mayhem as marchers departed from their approved route and split in different directions.

Ribbon-like columns of black-shirted protesters, kitted out in helmets, masks and goggles, snaked southwards from Mong Kok towards Yau Ma Tei, Jordan and then Tsim Sha Tsui.

More than half the shops along the route were shuttered as tourists gaped at the spectacle of demonstrators gathering metal fences to put up barricades at different streets in Mong Kok, a bastion protest site of the 2014 Occupy movement.

In Tsim Sha Tsui, near the clock tower, four protesters were spotted clambering up the flagstaff to remove the Chinese flag, which they flung into the harbour while tourists took pictures.

Nathan Road soon came to a standstill and protesters marched on towards the tunnel near Polytechnic University. Groups of them quickly emptied water barricades separating the two-way traffic and hauled them, along with metal fences, to block vehicles on both sides.

Traffic seized up for more than an hour, before protesters retreated as word spread that police were coming after them.

Protesters then drifted back to Mong Kok, some taking a break for dinner, while others appeared to be assessing where they ought to go next. The protesters were communicating verbally or in secret chat groups.

The usually crowded streets of Mong Kok, which would have been packed with diners and shoppers on a typical Saturday evening, were ceded over to them.

Several streets were barricaded with makeshift metal fences. At the junction of Waterloo and Nathan Road, protesters mounted bamboo poles from dismantled scaffolding as barricades and at least one street had plastic wrap spread across a part of the street to block users.

There was talk that police might deploy their latest anti-riot acquisition – water cannon trucks – after one such vehicle was seen on a road in Fanling out for testing.

A spokesman said there were strict guidelines for the trucks’ use and that they would only be deployed if there were “serious casualties, massive road blockage or destruction of public property”.

Past midnight, the government issued a statement condemning protesters who “deliberately undermine social peace and even challenge national sovereignty”. It accused protesters of destroying the national flag and violating the National Flag and National Emblem Ordinance.

After 1am on Sunday, police said in a webcast that the Wong Tai Sin crowd had destroyed public property and threatened the safety of residents. The force urged protesters to disperse immediately.

By 2am, groups of police were still stationed in the area but protesters had mostly cleared the scene, as with the stand-offs in Mong Kok and Tsim Sha Tsui.

Shops in the affected areas suffered from smaller takings because of the protests. A 75-year-old owner of a Mong Kok tuck shop, who wanted to be known only as Tse, said: “Once the hawkers heard about the protest, they packed away all the stalls. In the past 20 years, I’ve never seen the street so empty.”

He only made HK$7,000 (US$894) last month, he said, but needed to pay HK$8,000 for rent and around HK$2,000 more for electricity.

Police fire tear gas in Tsim Sha Tsui, Mong Kok as protesters fling petrol bombs

Also on Saturday, thousands of people showed their solidarity for police after Beijing gave a strong vote of confidence to the embattled force over the protest crisis.

The pro-establishment political group Politihk Social Strategic, which organised the rally, estimated 90,000 people attended the event themed “Give Peace A Chance” at the Victoria Park in Causeway Bay, while police put the figure at 26,000 at its peak.

The event came a day after Chinese state media ran interviews with three representatives of Hong Kong police unions, who insisted that the local force was able to handle the protests rocking the city and had so far used only minimal force.

The rally was reported by the state broadcaster CCTV in its evening news cast, which said the event was “peacefully conducted with music” and featured several interviews of participants who spoke Mandarin.

Another day of protests is expected on Sunday, with a march and rally planned for Tseung Kwan O and a gathering in Kennedy Town.

Reporting by Tony Cheung, Zoe Low, Rachel Cheung, Sarah Zheng, Su Xinqi, Victor Ting, Gigi Choy, Kanis Leung, Stuart Lau, Kimmy Chung and Danny Lee