Hong Kong police have arrested six men - some believed to have links to triad gangs - in connection to the violent mob attack on Sunday that saw dozens of masked men assault civilians returning home after a pro-democracy demonstration.

The onslaught over the weekend saw dozens injured and shocked the region as footage of the brutal assault quickly circulated online.

Senior police official Chan Tin-chu said the men, aged 24-54, were held for “unlawful assembly” and are being investigated for taking part in the attack late Sunday night. He added that some of them are villagers with occupations ranging from drivers and hawkers to renovation workers.

“Some of them have triad backgrounds,” he said. “I believe that more… will be detained soon. Police will not condone any form of violence.”

No additional details were provided.

Triads are organized criminal networks operating in Hong Kong, which are also known as the Chinese Mafia.

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A gang of white-clad men armed with metal rods and wooden poles assaulted pro-democracy demonstrators and others inside a subway station in Hong Kong Yuen Long neighborhood late Sunday, injuring 45 people, including a man who remained in critical condition. As the protesters made their way home, the white-clad men descended on the mob and charged into trains.

Video footage showed the violence as civilians attempted to defend themselves with umbrellas.

More than 100,000 people took part in the latest rally in the city earlier that day to demand democracy and an investigation into the use of force by police to disperse crowds at the summer-long protests. Some protesters on Sunday directed their anger at Beijing, pelting its office in Hong Kong with eggs, spray-painting a wall and defacing the Chinese national emblem.

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Police have come under fire for their slow response to the violence against the protesters, but Hong Kong’s Chief Executive Carrie Lam said police were stretched thin due to the massive crowds. However, she dodged accusations that her government and police force colluded with the assailants.

The assault escalated a crisis at the former British colony that has seen thousands of people protest against an extradition bill that would send criminal suspects back to China for trial. Critics view the bill as Beijing’s growing influence and fear it will chip away at Hong Kong’s freedoms promised under the “one country, two systems” formula since its handover back to China in 1997.

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The events have since attracted global attention as U.S. President Donald Trump commented on the conflict. Trump told reporters at the White House the unrest in Hong Kong was “very sad”.

“They’re looking for democracy and I think most people want democracy,” he said. “Unfortunately, some governments don’t want democracy.”

The Associated Press contributed to this report.