A group of thugs said to be triad gangsters attacked commuters at a subway station in Hong Kong following an anti-extradition bill protest late Sunday, leaving at least 45 people injured, with police criticized for failing to protect them.

Dozens of white-shirted people armed with cane and rods attacked commuters, thought to include protesters against the government bill, at the platforms of the Yuen Long MTR Station and in train compartments, video clips of the violence showed.

The hourlong incident saw the gang chasing and attacking people at random. Some victims suffered head injuries with blood all over their faces.

Democrat Party legislator Lam Cheuk-ting was among the injured commuters, having been hit in the head and mouth.

"They were armed, up to 100 of them," he told reporters. "Everyone in the station, those standing by the ticket machines, the stair railings, was being targeted."

"They chased us into the subway car, continued attacking us. Some recognized me and attacked me. The incident lasted for about an hour. It is infuriating that the police did not try to control the situation."

Journalists were also attacked.

A reporter for Stand News, who filmed an attacker coming at her during a live broadcast, sustained injuries to her head and back. A Now TV reporter was filming attackers before they turned and attacked him, smashing his camera.

A man who witnessed the incident told a radio program that police, after arriving on the scene, let the thugs, many of whom wore masks, leave right pass them, ignoring people's pleas for help.

Yuen Long district deputy commander Yau Nai-keung told the press that officers arriving at the scene did not see anyone holding weapons and said police could not arrest people merely on the basis of the color of their clothes.

He described the incident as a "brawl" between groups of people with different political ideas.

No arrests had been made as of midday Monday, though the police vowed to "seriously take enforcement actions."

"Are you saying that Yuen Long is an independent zone where laws in Hong Kong do not apply?" Civic Party lawmaker Alvin Yeung said in criticizing police inaction. "It is the triad gangsters attacking Hong Kong people, and you (police) call it a brawl?"

Pro-establishment camp lawmakers also condemned the violent attack, while giving harsher reprimand against protesters challenging China's authority at another demonstration.

The attack in Yuen Long, a rural area of the New Territories, happened around the same time as police were facing off protesters who rallied at Beijing's representative office following a mostly peaceful demonstration against the China extradition bill earlier Sunday on Hong Kong Island.

The protesters had thrown eggs at the plaque of the Central Government Liaison Office and ink at the emblem of China, while some sprayed graffiti on its walls to protest the encroachment on Hong Kong's freedoms.

"We offer our strongest condemnation against rioters who contained the office, defiled the national emblem and damaged the facility," office director Wang Zhimin told reporters.

"The blatant challenge to the rule of law, attack on the dignity of the central authorities and damage of people's interests and the feelings of all Chinese people are not to be tolerated.

(Riot police fire tear gas to disperse protesters in Hong Kong on July 21, 2019.)

Police fired tear gas, rubber bullets and bean bags at the crowd, who retreated subsequently in the small hours of Monday.

They said protesters, at a later standoff hurled bricks, smoke grenades and petrol bombs.

At a joint press conference with her chief ministers, Chief Executive Carrie Lam condemned the violent incidents at both the liaison office and in Yuen Long.

"We absolutely do not condone that sort of violent acts," Lam said. "Violence is not a solution to any problem. Violence will only breed more violence, and at the end of the day, the whole of Hong Kong and the people will suffer as a result of the loss of law and order in Hong Kong."

Police Commissioner Stephen Lo admitted that police officers responding to the Yuen Long attack were understaffed and underequipped, and he blamed the demonstrations throughout the day for the thinning of police manpower, while rejecting accusations that police collaborated with triad gangs in any operation.

"There was a series of violent confrontations during the public procession and definitely our manpower is stretched," Lo said.

"Because every time when there is a major event, which may lead to violent confrontation, we have to redeploy some of the manpower from various districts to the Hong Kong Island. What (the protesters) are doing is at the cost of the whole community."