Hong Kong police face criticism for an apparent failure to protect anti-government protesters and passersby from attack by suspected gang members at a train station on the weekend.

Sunday's attack came during a night of escalating violence that opened new fronts in Hong Kong's widening political crisis over an extradition bill that could see people sent to China for trial.

Earlier in the day, protesters surrounded China's main representative office in the city, defaced walls and signs, and clashed with police.

Hong Kong's Beijing-backed leader, Carrie Lam, condemned the attack on China's main office in the city, the Central Government Liaison Office, saying it was a "challenge" to national sovereignty.

She said she had been shocked by the clashes at the station, adding police would investigate fully. Allegations that police colluded with the train assailants are "unfounded," she said.

"Violence will only breed more violence," said Lam, flanked by senior city officials.

Activists appeared to be targets

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.

Snippet of a live broadcast from lawmaker Lam Cheuk ting, showing self-professed pro-Gov't mobsters attacking passengers in train cars at <a href="https://twitter.com/hashtag/MTR?src=hash&ref_src=twsrc%5Etfw">#MTR</a> <a href="https://twitter.com/hashtag/YuenLong?src=hash&ref_src=twsrc%5Etfw">#YuenLong</a> Stn. <a href="https://twitter.com/hashtag/HongKong?src=hash&ref_src=twsrc%5Etfw">#HongKong</a> has 1 of the world's highest cop to population ratio. Where were <a href="https://twitter.com/hkpoliceforce?ref_src=twsrc%5Etfw">@hkpoliceforce</a>? Lam was injured as shown live. <a href="https://t.co/Aq5JmJlf5u">pic.twitter.com/Aq5JmJlf5u</a> —@ray_slowbeat

On Sunday night, scores of men in white T-shirts, some armed with clubs, flooded into the rural Yuen Long station and stormed a subway train, attacking passengers with pipes, poles and other objects, according to video footage.

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

23:22 21 July: During the beating by suspected gangsters in Yuen Long Station, lawmaker Lam Cheuk-ting bled with injuries around his mouth.<br><br>Credit: Hong Kong Columns - Translated <br><br>(Photo: Lam Cheuk ting's Facebook live) <a href="https://t.co/y0vWpD5inf">pic.twitter.com/y0vWpD5inf</a> —@tanijo

Legislator Lam, who was wounded in the face and hospitalized, 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 told Reuters, saying the floors of the station were streaked with blood.

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

'Not a normal confrontation'

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

Hong Kong police Chief Stephen Lo, asked about concern police had been slow to respond to the clash at the station, said there had been a need to "redeploy manpower from other districts."

Witnesses said the men in the white T-shirts appeared to target black-shirted passengers who had been at an anti-government march. (Tyrone Siu/Reuters )

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

"We will pursue at all costs to bring the offenders to justice," he told reporters, pledging to restore public confidence in the police force.

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

Triad suspects traced to nearby village

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.

Protesters are engulfed by tear gas during a confrontation with riot police in Hong Kong on Sunday. (Lo Kwanho/HK01 via AP)

"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.

Chinese troops hold exercise nearby

A Chinese army brigade, meanwhile, says it has held what it calls an anti-terrorism exercise in a city not far from Hong Kong.

A social media post Monday by the 74th Army Group did not refer to ongoing protests in Hong Kong, but military commentator and retired officer Yue Gang said the exercises show that troops would be dispatched to the semi-autonomous territory if needed.

The property damage to China's office in Hong Kong, on the seventh consecutive weekend of demonstrations, has fuelled fears that the People's Liberation Army may intervene.

The state-owned Global Times newspaper said the exercise in Guangdong province's Zhanjiang city simulated a response to an outbreak of violence and terrorism.

In 2014, Hong Kong's anti-triad police units 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.

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

"I hope that the police will not deceive themselves," Yeung said. "It is a triad fight, and not a normal confrontation."

Rubber bullets

Hong Kong has been rocked by a series of sometimes violent protests for more than two months in its most serious crisis since Britain handed the Asian financial hub back to Chinese rule in 1997.

Under terms of the handover, 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.

Many city residents fear the proposed extradition law, which would allow people to be extradited to mainland China for trial, where the courts are controlled by the Communist Party, would undermine Hong Kong's judicial independence.

Hong Kong's government, responding to the scale of the protests, postponed it and later said it was "dead," but the protesters are demanding its full withdrawal and calling on Lam to resign — something she has declined to do.

Internet video shows triad members in Yuen Long of <a href="https://twitter.com/hashtag/HongKong?src=hash&ref_src=twsrc%5Etfw">#HongKong</a> gather & pose threat to peaceful protestors. <br><br>How long <a href="https://twitter.com/hashtag/HKSAR?src=hash&ref_src=twsrc%5Etfw">#HKSAR</a> government & <a href="https://twitter.com/hkpoliceforce?ref_src=twsrc%5Etfw">@hkpoliceforce</a> can go to tolerate these triad members? Have they got a non-objection notice of gathering? <a href="https://t.co/RzqoN4CJHZ">pic.twitter.com/RzqoN4CJHZ</a> —@CraigChoy

They are also demanding independent inquiries into the use of the police against protesters. Some are also demanding full democracy — anathema to Beijing's party leadership.

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.

Eggs and spray paint

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 spokesperson said such acts tested Beijing's limits.

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

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