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Violence erupted in Hong Kong last night as dozens of masked men stormed a train station and attacked protesters.

The men, suspected triad gangsters, flooded into the rural Yuen Long station and stormed a train where they launched a terrifying assault, footage has shown.

Eye-witnesses said white-shirted assailants appeared to target black-shirted passengers who had been at an anti-government march.

Footage posted on social media showed dozens of men attacking people with batons inside the station.

Hong Kong Protests: Yuen Long Station Attack - In pictures 15 show all Hong Kong Protests: Yuen Long Station Attack - In pictures 1/15 Men in white t-shirts and face masks attack anti-extradition bill demonstrators and reporters at a train station in Hong Kong Stand News via REUTERS 2/15 Men in white t-shirts and face masks attack anti-extradition bill demonstrators and reporters at a train station in Hong Kong 3/15 Men in white t-shirts and face masks attack anti-extradition bill demonstrators and reporters at a train station in Hong Kong 4/15 A mob of men in white T-shirts clashing with pro-democracy protesters on the platform of Yuen Long station in Hong Kong AFP/Getty Images 5/15 Men in white t-shirts and face masks attack anti-extradition bill demonstrators and reporters at a train station in Hong Kong 6/15 a mob of men in white T-shirts threatening pro-democracy protesters (out of frame) after clashes between the two groups at Yuen Long station in Hong Kong AFP/Getty Images 7/15 Men attacking pro-democracy protesters (out of frame at right) on a train on the platform of Yuen Long station in Hong Kong AFP/Getty Images 8/15 Men in white T-shirts (right) attacking pro-democracy protesters at a train at Yuen Long station in Hong Kong following earlier clashes AFP/Getty Images 9/15 A mob of men in white T-shirts threatening pro-democracy protesters (out of frame at R on train) after chasing them following clashes between the two groups at Yuen Long station in Hong Kong AFP/Getty Images 10/15 A man in a white T-shirt threatening pro-democracy protesters (bottom R-on train) on the train platform following clashes between the two groups at Yuen Long station in Hong Kong AFP/Getty Images 11/15 A mob of men in white T-shirts attacking pro-democracy protesters on the platform of Yuen Long station in Hong Kong AFP/Getty Images 12/15 Protesters clash with police after taking part in an anti-extradition bill march Getty Images 13/15 Protesters clash with police after taking part in an anti-extradition bill march Getty Images 14/15 Protesters clash with police after taking part in an anti-extradition bill march Getty Images 15/15 Protesters clash with police after taking part in an anti-extradition bill march Getty Images 1/15 Men in white t-shirts and face masks attack anti-extradition bill demonstrators and reporters at a train station in Hong Kong Stand News via REUTERS 2/15 Men in white t-shirts and face masks attack anti-extradition bill demonstrators and reporters at a train station in Hong Kong 3/15 Men in white t-shirts and face masks attack anti-extradition bill demonstrators and reporters at a train station in Hong Kong 4/15 A mob of men in white T-shirts clashing with pro-democracy protesters on the platform of Yuen Long station in Hong Kong AFP/Getty Images 5/15 Men in white t-shirts and face masks attack anti-extradition bill demonstrators and reporters at a train station in Hong Kong 6/15 a mob of men in white T-shirts threatening pro-democracy protesters (out of frame) after clashes between the two groups at Yuen Long station in Hong Kong AFP/Getty Images 7/15 Men attacking pro-democracy protesters (out of frame at right) on a train on the platform of Yuen Long station in Hong Kong AFP/Getty Images 8/15 Men in white T-shirts (right) attacking pro-democracy protesters at a train at Yuen Long station in Hong Kong following earlier clashes AFP/Getty Images 9/15 A mob of men in white T-shirts threatening pro-democracy protesters (out of frame at R on train) after chasing them following clashes between the two groups at Yuen Long station in Hong Kong AFP/Getty Images 10/15 A man in a white T-shirt threatening pro-democracy protesters (bottom R-on train) on the train platform following clashes between the two groups at Yuen Long station in Hong Kong AFP/Getty Images 11/15 A mob of men in white T-shirts attacking pro-democracy protesters on the platform of Yuen Long station in Hong Kong AFP/Getty Images 12/15 Protesters clash with police after taking part in an anti-extradition bill march Getty Images 13/15 Protesters clash with police after taking part in an anti-extradition bill march Getty Images 14/15 Protesters clash with police after taking part in an anti-extradition bill march Getty Images 15/15 Protesters clash with police after taking part in an anti-extradition bill march Getty Images

Forty-five people were injured, with one person in critical condition.

But police were slow to arrive at the scene, sparking questions from law-makers.

Democratic Party lawmaker Lam Cheuk-ting, who was injured in the attack, said it took police more than an hour to arrive after he alerted them.

He said officers had failed to protect the public and allowed the triads to run rampant, according to government-funded broadcaster RTHK.

"Is Hong Kong now allowing triads to do what they want, beating up people on the street with weapons?," he asked reporters.

While fellow pro-democracy lawmaker Ray Chan asked in a tweet: "Hong Kong has one of the world's highest cop to population ratio. Where were [they?]"

A government worker, aged 22, described chaotic scenes as people scrambled off the train: "People started to run in all directions, just avoiding the gangsters," he said.

Police said they had not made any arrests but were still carrying out investigations.

The masked men stormed Yuen Long MTR station at about 10.30pm local time (2.30pm UK time), as people made their way back from protests in Hong Kong’s financial district.

The government said in a statement that "some people” congregated at the platforms of the Yuen Long station and train compartments, “attacking commuters".

"This is absolutely unacceptable to Hong Kong as a society that observes the rule of law. The SAR (Special Administrative Region) Government strongly condemns any violence and will seriously take enforcement actions," it added.

Hong Kong Police also confirmed: "Some people attacked commuters at the platforms of the Yuen Long MTR station and train compartments, resulting in multiple injuries."

The mob attack followed a pro-democracy rally earlier on Sunday in the centre of Hong Kong, during which riot police fired tear gas and rubber bullets at protesters.

Organisers of Sunday's protest say more than 430,000 people took part but police put the figure at 138,000.

Hong Kong protests reignite in Sheung Shui - in pictures 14 show all Hong Kong protests reignite in Sheung Shui - in pictures 1/14 Several thousand people are marching in Hong Kong against traders from mainland China in what is fast becoming a summer of unrest. AP 2/14 Police swing their batons at protesters as they try to disperse pro-democracy activists after a march in the town near the Chinese border. REUTERS 3/14 Protesters chant slogans outside a pharmacy during a march at an anti-parallel trading in Sheung Shui district. AFP/Getty Images 4/14 A photojournalist falls down during clashes between protesters and police at the anti parallel trading march. AFP/Getty Images 5/14 Protesters highlighted the local issue of Chinese parallel importers who buy products in Sheung Shui tax-free, and sell them for a profit in mainland China. EPA 6/14 Police officers use pepper spray while protesters protect themselves with umbrellas. Getty Images 7/14 Protesters hold up words that read: "Strict enforcing of law against smugglers of grey goods" in Hong Kong AFP/Getty Images 8/14 Protesters carried slogans as they took to the streets. EPA 9/14 A supporter begs police officer not to attack protesters. AP 10/14 Protesters braved the rain in the Sheung Shui district. EPA 11/14 Policemen clash with protesters EPA 12/14 Police try to disperse the pro-democracy activists REUTERS 13/14 Police scuffle with Hong Kong protesters again. AP 14/14 Several thousand people are marching in Hong Kong against traders from mainland China in what is fast becoming a summer of unrest in the semi-autonomous Chinese territory. AP 1/14 Several thousand people are marching in Hong Kong against traders from mainland China in what is fast becoming a summer of unrest. AP 2/14 Police swing their batons at protesters as they try to disperse pro-democracy activists after a march in the town near the Chinese border. REUTERS 3/14 Protesters chant slogans outside a pharmacy during a march at an anti-parallel trading in Sheung Shui district. AFP/Getty Images 4/14 A photojournalist falls down during clashes between protesters and police at the anti parallel trading march. AFP/Getty Images 5/14 Protesters highlighted the local issue of Chinese parallel importers who buy products in Sheung Shui tax-free, and sell them for a profit in mainland China. EPA 6/14 Police officers use pepper spray while protesters protect themselves with umbrellas. Getty Images 7/14 Protesters hold up words that read: "Strict enforcing of law against smugglers of grey goods" in Hong Kong AFP/Getty Images 8/14 Protesters carried slogans as they took to the streets. EPA 9/14 A supporter begs police officer not to attack protesters. AP 10/14 Protesters braved the rain in the Sheung Shui district. EPA 11/14 Policemen clash with protesters EPA 12/14 Police try to disperse the pro-democracy activists REUTERS 13/14 Police scuffle with Hong Kong protesters again. AP 14/14 Several thousand people are marching in Hong Kong against traders from mainland China in what is fast becoming a summer of unrest in the semi-autonomous Chinese territory. AP

Mass protests have been held for weeks, initially over an extradition deal with mainland China but now covering other issues on democracy in Hong Kong.

Protesters are demanding the full withdrawal of a bill to allow people to be extradited to mainland China for trial, where the courts are controlled by the Communist Party, fearing it would undermine Hong Kong's judicial independence.

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

Some police had been injured in the clashes after protesters hurled bricks, smoke grenades and petrol bombs, the force said in a statement.