The organiser of the Mong Kok rally said about 120,000 attended. On Sunday, protests were planned in Tseung Kwan O and the west of Hong Kong Island, both densely populated residential districts.

On Saturday night, clashes flared in Wong Tai Sin after reports that the police had arrived to arrest two people. Hundreds blocked police vans from leaving, and attacked outnumbered officers with fire extinguishers, helmets, umbrellas and other objects.

At about 10.20pm, some protesters who had ventured to Kowloon City from Mong Kok started to march along Prince Edward Road to Wong Tai Sin Police Station, where reporting room services were soon suspended.

Riot police used tear gas, pepper spray and batons in the streets of the densely packed neighbourhood, prompting many residents – some of them, such as elderly, shirtless men, without protective gear – to join the protesters in turning on officers, accusing them of disrupting their community.

“Wong Tai Sin does not welcome you, go away!” some of them shouted.

At midnight, police raised the black flag, firing tear gas about 10 minutes later. According to police, some protesters also threw objects near the disciplined services quarters, where officers and their families live.

At one point, chaos broke out among residents near the services quarters, with people from both sides attacking each other with wooden sticks and umbrellas. The scuffles were soon stopped by other residents.

Amid the chaos, a tear-gas canister thrown into the crowd was promptly extinguished using an aluminium dish of the sort commonly used to steam fish.

Protesters also besieged Wong Tai Sin Police Station, vandalising its CCTV system, while at the same time, police in Mong Kok continued to fire tear gas at the remaining crowds.

The stand-off in Wong Tai Sin lasted until around 4am, with most of the protesters leaving after more rounds of tear gas were fired.

During a briefing early on Sunday, police condemned violent protesters and said their behaviour had pushed officers to ultimately disperse the crowd using minimal force.

How police fired tear gas in Tsim Sha Tsui, Mong Kok as protesters hurled petrol bombs

The 20 or so protesters were arrested on suspicion of unlawful assembly, possession of a dangerous weapon and assault.

“Radical protesters deliberately spread their violent behaviour to several areas in eastern Kowloon, including residential areas. Besides blocking many main thoroughfares in Kowloon, they were also risking the safety of many innocent citizens,” Senior Superintendent Yolanda Yu Hoi-kwan said.

When reporters questioned the police’s use of flashlights which disrupted filming during the protest, Yu said the lights were used for illumination and for identifying protesters in the dark.

In a statement, the government vowed to bring the radical protesters to justice.

“The government strongly condemns the radical protesters who disregarded the law and order, blatantly breached the public peace, and even challenged national sovereignty. The police will strictly enforce the law. Violent protesters who breached the law should also be brought to justice,” it said.

“In addition, protesters were suspected to have deliberately damaged the national flag, breaching the National Flag and National Emblem Ordinance.”

On Saturday, near the clock tower in Tsim Sha Tsui, four protesters were spotted clambering up a flagstaff to remove the Chinese national flag, which was then flung into the water in Victoria Harbour while tourists took pictures.

“We express regret over such behaviour which is illegal and disregards public order and the needs of other members of the public. Indeed, illegal behaviour and road blockages involved in recent protests, held in the name of expression of aspirations, are getting worse and go far beyond the boundary of freedom of expression in a civilised society,” the government statement said.