The protests in Hong Kong over reforms have taken a new twist with reports of attacks by suspected triad gang members on demonstrators. The assaults were also mounted on commuters at a railway station last weekend.

Police, widely slated for their inability to protect the people from club-wielding men in Yuen Long, had refused to allow the march in the town on grounds of safety.

Demonstrators are seen amidst smoke from tear gas during a protest in Yuen Long, Hong Kong. Edgar Su/Reuters

But protesters pushed ahead and what began as a silent action by several thousand in sweltering afternoon heat soon spiralled into a tense stand-off between police and protesters in several locations.

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Rocks and bottles were thrown at police by protesters, who were also building barricades out of street furniture and umbrellas. Police responded with tear gas.

"I forgot my umbrella, so I had to buy this badminton racket ... just for self-protection,” said one young helmeted, masked man, who declined to give his name or age as he sped to the front line.

The protester added that many who marched did not want to stay late, saying Yuen Long was too dangerous for them after dark.

Last Sunday, about 100 white-shirted men stormed the Yuen Long mass-transit station hours after protesters marched through central Hong Kong and defaced China's Liaison Office - the main symbol of Beijing's authority over the former British colony.

The men attacked black-clad protesters returning from Hong Kong island, passers-by, journalists and lawmakers with pipes and clubs, leaving 45 people injured.

Police, considered slow by protesters to respond last Sunday, became a focus for Saturday's march, adding to the tensions.

"They failed the public," a protester called Kevin, in a red T-shirt, said of the police earlier in the afternoon, as he stood outside the police station, gripping its gates.

Police officers fire tear gas at demonstrators during a protest in Yuen Long, Hong Kong. Tyrone Siu/Reuters

"They deliberately let the triads beat up protesters to get revenge on us... We're here to teach them a lesson," he said, as he shouted an obscenity at the police.

Police said on Sunday some protesters had surrounded and vandalised a police van, "causing danger to the life of the police officers onboard."

The statement, issued at 5.38 pm (0938 GMT), went on to warn protesters police would soon begin dispersing the crowds.

Protests intensify

The Yuen Long attack and the vandalism at the Liaison Office marked new fronts in a protest movement that has intensified over the last two months.

The protests, considered the most direct challenge to the authority of China's President Xi Jinping, mushroomed on Friday as thousands of activists thronged the arrivals halls of Hong Kong international airport.

The protesters, initially demanding the scrapping of a bill that would have allowed people to be sent to mainland courts for trial, are now also seeking independent inquiries into police use of force, the resignation of Hong Kong leader Carrie Lam and full democratic reform — anathema to Beijing's Communist Party leadership.

Demonstrators clash with police during a protest against the Yuen Long attacks in Yuen Long, Hong Kong. Reuters

The crisis is exposing fissures in Lam's administration, with police chiefs and rank-and-file officers enraged at an apology over last weekend's attacks by her chief secretary on Friday, apparently made without consultation.

The official, Matthew Cheung, said the government would not shirk its responsibility "and the police's handling fell short of residents' expectations".

Britain handed Hong Kong to China in 1997 amid guarantees that its core freedoms and autonomy, including the right to protest and an independent judiciary, would be protected under a "one country, two systems" formula.

Protesters hold up a poster with the words "Stop violence" as they face off with riot police in Hong Kong .

Many fear those rights are under threat as Beijing's reach extends into the city.

Activists told Reuters they feared Saturday's protest would turn violent, given anger among the protesters over last Sunday's violence and a determination among some to challenge villagers they believe are close to triad groups in the area.

"We are hoping for a peaceful night," said Neil, masked, in his mid-20s, standing next to a friend who was strapping on a hard hat.

"We want Yuen Long to be safe and peaceful. But there still might be trouble so we have to be prepared."

Several banks in the area did not open on Saturday and many businesses were shuttered.

Reuters