Violence escalated to a new level in Hong Kong on Sunday night, when men dressed in white left protesters beaten and bloodied at a train station.

Key points: Triads are organised crime groups that are sometimes described as "mafia-like"

Triads are organised crime groups that are sometimes described as "mafia-like" Triads run protection rackets for the illicit drug trade as well as legal businesses

Triads run protection rackets for the illicit drug trade as well as legal businesses Protesters have accused police of turning a blind eye to the brutal attackers

Reports said police arrested up to 11 alleged perpetrators with "triad backgrounds" after the attack at Yuen Long — an area known for gang activity.

Warning: This story contains images which some people may find distressing.

But what are triads, and why would they be involved in this violent new twist in Hong Kong's pro-democracy protests?

What are triads?

Essentially, they are organised crime groups — secret societies that are sometimes described as "mafia-like".

Triad is an English word, and in Chinese they're referred to as "dark societies" or "black societies", according to criminology professor Roderic Broadhurst, a triad expert at the Australian National University.

Calvin So was beaten during the Yuen Long attacks on Sunday. ( Reuters: Tyrone Siu )

They originally stem from the mid-17th century, when a secret group loyal to the ousted Ming emperor wanted to overthrow the Qing dynasty.

These triads "devolved from nascent political associations into criminal groups" and "quickly transformed into being social bandits", Professor Broadhurst told the ABC.

"It's a bit like having bogans run the state, but they don't actually run it — they use political proxies," he said.

"Their hidden power, their hidden wealth can really undermine government."

Triads are a staple of Hong Kong martial arts movies, which feature initiation ceremonies like blood rituals, chicken sacrifice and the 36 oaths — a series of promises to the brotherhood which, if broken, will result in "death by five thunderbolts".

Protesters threw black paint on the National Emblem of the People's Republic of China. ( AP: Bobby Yip, file )

But Professor Broadhurst says such rituals are mostly "museum pieces" or, when rejuvenated to bolster their underlying mythology, occur in secret.

Today's triads, like the fractured 14K and Wo Shing Wo, run protection rackets for both illicit and legal markets — everything from drugs, prostitution and gambling; to restaurants, nightclubs, film production and even funeral services.

"There isn't an area of Hong Kong life that isn't in some way affected by the activities of these groups," he said.

"We've often thought the triad battle has been largely won, but actually in practice they surfaced again in the Umbrella Movement," he said.

Are they thugs for hire?

Protesters in black shirts, as well as commuters, were left bloodied and bruised after the attack. ( AP via Apple Daily. )

The triads have been accused of carrying out Beijing's "dirty work" in Hong Kong.

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Max Chung, a Yuen Long local and the organiser for this Saturday's "Take Back Yuen Long" protest, suspected some may have come from mainland China.

"They looked like triad gangsters. Most of them are local but some don't speak [Cantonese], the local language," he said.

This isn't the first time triads have arisen in pro-democracy protests. Five years ago, peaceful activists were set upon by suspected triad members during the Occupy and Umbrella movements of 2014.

At the time, academics Federico Varese and Rebecca Wong suggested triads "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".

After the Tiananmen Square massacre of 1989 and the British handover of Hong Kong to China, Beijing used a "united front" tactic to recruit "patriotic" triads to the communist camp, City University of Hong Kong researchers say.

Professor Broadhurst said some traits seen in footage from the attacks were "pretty consistent" with the behaviour of triad groups.

"They may not all be badge-wearing, signed-up triads — they might be associates, and that's not unusual," he said, adding the attacks could have been "spontaneous" rather than orchestrated by the state.

The white clothing worn, while partly to distinguish them from protesters wearing black, was symbolic of mourning or death, he added.

The attacks effectively struck fear into would-be protesters and worked to intimidate pro-democracy activists, he added.

"The cynic might argue that violence was allowed to escalate, and that served the purposes of the state," Professor Broadhurst said.

Did police 'let it happen'?

Men in white talk to riot police in Yuen Long after attacking anti-extradition bill demonstrators. ( Reuters: Tyrone Siu )

There are some lingering question marks about whether the men in white were really linked to triads, but a recurring question from the attacks is — where were the police?

Kenneth Chan, a politics professor at Hong Kong Baptist University, told the ABC that officials in Hong Kong had met with village heads in Yuen Long and urged them to defend "their home" against the protesters.

Hong Kong Chief Executive Carrie Lam condemned both the attackers and the protesters. ( AP: Vincent Yu )

He said it was unusual that police didn't take pre-emptive action, when many Hongkongers got tip-offs that gangsters were planning an attack.

"The can't-care-less attitudes and sluggish responses to people's calls for help during the rampage were taken as prima facie evidence by people that the police 'let it happen'," he said.

Hong Kong's embattled Chief Executive, Carrie Lam, condemned the attackers at a press conference on Monday, according to the Straits Times.

"We do not condone any violence," she said.

"We witnessed the lawlessness and wilful hurt of citizens and train passengers by the attackers in Yuen Long Station. Their acts are hateful."

Hong Kong police chief Stephen Lo on Monday denied police had given attackers free rein and said forces were stretched thin on the night of the violence.

Hong Kong police told the ABC the commissioner had dismissed any suggestion the police colluded with triads.

China's Ministry of Foreign Affairs has been contacted for comment.