Hong Kong (CNN) Six men have been arrested in Hong Kong in connection with a seemingly unprovoked and indiscriminate mob attack that injured 45 people at a metro station on Sunday night and outraged a city not used to such violent scenes.

The men, whose ages range from 24 to 54, will face charges of "unlawful assembly" police said in a press conference on Monday. Their motives are still under investigation but police said some of those arrested had links to local criminal gangs known as Triads.

Footage posted on social media Sunday night showed a marauding gang of masked men, wearing white T-shirts and wielding batons and sticks, blindly attacking crowds on the platform and inside train carriages at Yuen Long MTR station, in the far northwest of the city.

Tens of thousands had taken to the streets Sunday for the seventh consecutive weekend, amid an ongoing political crisis over a now-suspended extradition bill.

Many of those caught up in the violence were returning home after taking part in mass demonstrations in the city, leading to accusations that the gangs had been paid to stoke unrest and target protesters.

Forty-five people were hospitalized following the violence in Yuen Long, with one person in critical condition, according to Hong Kong's Information Services Department. Videos showed people being beaten on the floor and left bloodied and dazed.

The incident angered many in the city, a feeling that was exacerbated by police taking around an hour to arrive on scene and not making any arrests that night. The nature and ferocity of the attack also has people questioning Hong Kong police's ability to protect the city's residents.

Protesters have vowed to march in Yuen Long on Sunday to protest the violence.

What, or who, are Triads?

Men in white T-shirts with poles are seen in Yuen Long after attacking anti-extradition bill demonstrators at a train station, in Hong Kong.

The six men were detained after raids on their homes in Yuen Long and nearby Tin Shui Wai districts -- which are close to the Chinese border -- on Monday, according to the New Territories North Regional Crime Unit.

The suspects include drivers, vendors, renovation workers and the unemployed and police said some had "Triad backgrounds."

Triad is a name given to Hong Kong's organized crime syndicates that make their money through illicit drugs, gambling and prostitution among other activities. It has also become to refer to more loosely organized criminal gangs in the city. Some groups, especially the bigger, more powerful gangs who operate in the rural New Territories villages, wield political power through district councils and other political connections.

Pro-democracy lawmaker Claudia Mo said on Facebook that the scenes in Yuen Long were evident of a "more than apparent collusion between the police and the Triads in that neighborhood."

Police Commissioner Stephen Lo on Monday denied accusations that law enforcement officials were working with gangs hired to attack protesters and said the delay was because police resources were deployed to the main protest site on Hong Kong island, about an hour away from Yuen Long.

"We will investigate whether we were inefficient but we are not related to triads. I ask you to trust the police force. Last night, we were all focusing on Hong Kong island. We needed to regroup for Yuen Long," he said.

Hong Kong's Chief Executive Carrie Lam called such accusations "insulting."

Thugs for hire

But similar claims of collusion were made in 2014, when masked men with alleged links to organized crime attacked Umbrella Movement protesters who had occupied the city's Mong Kok district for weeks. Protesters said police failed to protect them and did not arrest people seen committing violence, a charge the force denied at the time.

In their study "Resurgent Triads? Democratic mobilization and organized crime in Hong Kong," researchers concluded that the 2014 Mong Kok attackers were "low-level Triads affiliates" that were paid to attack the protesters.

The researchers Federico Varese from the University of Oxford and Rebecca WY Wong City University Hong Kong said Triad groups "might have found a new role as enforcer of unpopular policies and repression of democratic protests in the context of a drift towards authoritarianism in Hong Kong."

Professor T Wing Lo, expert in triad societies at City University of Hong Kong, told CNN that the level of organization in the scenes from Sunday suggest it was carried out by by a Triad group.

"But the participants were not all triad members," he said. "Some were just ordinary villagers, some were paid to do a job."

Lo said the group who carried out the attack in Yuen Long would likely have been paid by Chinese authorities.

"All the crime committed by Triads is for money," Lo said. "We call this extralegal governance -- sometimes when governments cannot use the formal law enforcement for whatever reason, they pay for it. This is the normal way to do business."

While police have not confirmed that the incident in Yuen Long was coordinated under the direction of a Triad group, the phenomenon of thugs-for-hire is common across the border in mainland China.

Authorities are known to employ criminals to impose policies or enforce decisions, according to Lynette Ong, Professor of Political Science at the University of Toronto and author of "'Thugs-for-Hire': State Coercion and "Everyday Repression" in China."

"Third-party violence is commonly deployed by the state to evict homeowners and to deal with petitioners and protestors in China," Ong said in her 2015 paper.

That alleged link between Hong Kong Traid groups and mainland Chinese authorities goes back decades.

Following the Tiananmen Square massacre in 1989 and some Triad groups offering to help smuggle fleeing democracy leaders, Beijing embarked on a "deliberate strategy to "woo the Triads into the pro-Beijing camp," which included granting business opportunities to leaders in exchange for support in Hong Kong.

"Preparing for the island handover to China in 1997, the Beijing government was worried that Triads societies, and in particular the most powerful Sun Yee On Triad, would side with liberal political activists and destabilize post-1997 Hong Kong," Varese and Wong said.