Hongkongers are pointing fingers at the police for failing to protect pro-democracy protestors and bystanders attacked by armed groups on the weekend.

Key points: A group of masked men attacked protesters and bystanders in a subway on Sunday

A group of masked men attacked protesters and bystanders in a subway on Sunday Some have linked them to triad groups who attacked democracy protesters in 2014

Some have linked them to triad groups who attacked democracy protesters in 2014 Hong Kong police have been criticised for not arresting any of the attackers

The attack on Sunday came during a night of escalating violence that opened new fronts in Hong Kong's widening political crisis over an extradition bill, which has since spoken to wider existential concerns about the future of Hong Kong's freedoms.

Protesters had earlier surrounded China's main representative office in the city and defaced walls and signs and clashed with police — a move that Chinese state media has strongly condemned.

The city's Beijing-appointed leader, Carrie Lam, also condemned the attack on China's main office, and said it was a "challenge" to national sovereignty.

Ms Lam denied allegations that the police colluded with the masked assailants, of whom none were arrested. ( AP: Vincent Yu )

She condemned violent behaviour of any kind and said she had been shocked by the clashes at the station, adding police would investigate fully.

"Violence will only breed more violence," Ms Lam said.

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Some politicians and activists have linked Hong Kong's shadowy network of triad criminal gangs to political intimidation and violence in recent years, sometimes against pro-democracy activists and critics of Beijing.

Video released on Sunday night showed scores of men in white T-shirts, some armed with clubs, flooding into the rural Yuen Long station, storming a train, attacking passengers with pipes, poles and other objects.

Witnesses, including Democratic lawmaker Lam Cheuk-ting, said the men appeared to target black-shirted passengers who had been at an anti-government march.

Mr Lam, who was wounded in the face and hospitalised, said the police ignored calls he made, pleading with them to intervene to prevent bloodshed.

"They deliberately turned a blind eye to these attacks by triads on regular citizens," he said.

"I won't speculate on why they didn't help immediately."

'It is a triad fight, not a normal confrontation'

Some protesters were left with substantial injuries following the attack. ( AP via Apple Daily )

Forty-five people were injured in the violence at the station, with one in critical condition, according to hospital authorities.

Hong Kong's police chief Stephen Lo, asked why police had been slow to respond to the clash at the station, said there had been a need to "redeploy manpower from other districts".

Police stations nearby had closed given the risk of unrest, and a patrol on the scene needed to wait for reinforcements, he said.

In a video released by a pro-democracy activist, a police commander is heard telling a Radio Television Hong Kong (RTKH) reporter that he didn't realise he was late because he didn't get the chance to look at his watch.

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When pressed on why he wasn't able to detail when his unit arrived on the scene, he said:

"You think I'm scared? Of journalists? Come on."

Asked by a reporter if police had colluded with triads at the station, Mr Lo said the force had no links to triads.

Witnesses saw groups of men in white with poles and bamboo staves at a nearby village but police later found no weapons and allowed the men to leave without making any arrests.

"We can't say you have a problem because you are dressed in white and we have to arrest you," said Yau Nai-keung, an assistant police commander in the area.

"We will treat them fairly no matter which camp they are in."

Some banks and shops in the area closed early on Monday amid fears of more trouble.

Hong Kong's anti-triad police units in 2014 investigated the role of triad gangs attacking protesters during the pro-democracy demonstrations that shut down parts of the city for 79 days that year.

Riot police were pictured speaking to men in white t-shirts accused of attacking pro-democracy protesters. ( Reuters: Tyrone Siu )

Alvin Yeung, a barrister and lawmaker with the pro-democracy Civic Party, said he was sure the men were from gangs.

"I hope that the police will not deceive themselves," Mr Yeung said.

"It is a triad fight, and not a normal confrontation."

Beijing stares down latest Hong Kong unrest

State newspaper Xinhua said that defacing China's national emblem "caused unbearable pain and anger". ( AP: Bobby Yip, file )

Hong Kong has been rocked by its most serious crisis since Britain handed the Asian financial hub back to Chinese rule in 1997.

Under the terms of the Sino-British Joint Declaration of 1984, Hong Kong was allowed to retain extensive freedoms not enjoyed on the mainland under a "one country, two systems" formula, including an independent judiciary and the right to protest for a period of 50 years.

In 2017, the 20th anniversary of handover, Beijing said the document was not binding and had "no longer any practical significance".

Many city residents fear the proposed extradition law, which would allow extraditions with other jurisdictions including that of mainland China, would undermine Hong Kong's judicial independence.

Critics of the law also alleged that it would make it far easier for Beijing to silence its critics in Hong Kong.

The city's Beijing-backed government, responding to the scale of the protests, have not officially withdrawn it from the legislature, but Chief Executive Lam said the bill was "dead".

There have been numerous calls on her to resign.

Protestors have begun donning gas masks to minimise the effects of tear gas thrown by police. ( AP via HK01: Eric Tsang )

A political manifesto first revealed when protesters stormed Hong Kong's Legislative Council was re-iterated by protesters.

Hong Kong's pro-democracy manifesto Universal suffrage

Universal suffrage Direct voting rights for all Hong Kong residents

Direct voting rights for all Hong Kong residents Dropping charges against anti-extradition protesters

Dropping charges against anti-extradition protesters Stopping the clashes between protesters and police being labelled a "riot"

Stopping the clashes between protesters and police being labelled a "riot" Dissolving the Legislative Council

They are also demanding independent inquiries into the use of the police against protesters.

On Sunday, police fired rubber bullets and tear gas to disperse activists after thousands had ringed Beijing's Liaison Office.

The police said in statement that protesters hurled bricks, smoke grenades and petrol bombs during the unrest that came after hundreds of thousands marched through the city streets.

The Chinese government, including office director Wang Zhimin, condemned the turmoil, which included spray-painting and hurling eggs at walls and a national emblem at the office, saying the behaviour challenged the "authority and dignity" of the Chinese government.

A foreign ministry spokesman said such acts tested Beijing's limits.

"Some radical protestor behaviour violated our bottom line of 'one country, two systems'. We cannot tolerate that," said foreign ministry spokesman Geng Shuang.

The unrest in Hong Kong marks the greatest popular challenge to Chinese leader Xi Jinping since he came to power in 2012.

ABC/Reuters