Activists had planned to hold a protest in the rural town of Yuen Long where the brazen assaults took place
The savage beating of Hong Kong protesters by a mob of triad gangsters has deepened fears about the city's notorious criminal gangs and the use of shadowy hired muscle to defend China's interests.
At least 45 people were hospitalised after Sunday's attack when men armed with poles and rods assaulted anti-government protesters in the rural town of Yuen Long as they returned from another huge rally.
In a city that prides itself on a reputation for safety the scenes caused outrage and fear.
They also offered a glimpse into an underworld that has long stalked the city.
For seasoned watchers of Hong Kong and its shadowy nexus of organised crime, the brazen beatings came as little surprise.
In recent years, analysts say, there has been a growing tendency for thugs to target perceived threats to Beijing -- in Hong Kong, on the Chinese mainland and even in Taiwan.
"When the Communist Party of China finds it inconvenient for it to do something, it will use those gangsters on its behalf," local Hong Kong political analyst Dixon Sing told AFP.
- From enemies to bedfellows -
The idea of triads embracing Beijing's goals is a remarkable transition for crime groups that once used to be arch enemies of communism.
Their shifting allegiances reflect the malleable loyalties of organised crime and China's rising power.
Triads trace their origins to 19th century Chinese fraternal organisations.
Most fled to Hong Kong, Macau and Taiwan when the communists took power in mainland China in 1949 and revolutionary leader Mao Zedong cracked down on "black societies".
Many allied with the defeated Kuomintang Chinese nationalists in Taiwan.
The nationalists used them against their opponents during the nearly four decades of martial law known as the "White Terror" when thousands were killed of disappeared.
But many triad groups began shifting allegiances in the early 1990s as China embarked on its astonishing economic rise.
Beijing began showing an openness to triads during this time.
"As long as these people are patriotic, as long as they are concerned with Hong Kong's prosperity and stability, we should unite with them," Tao Siju, chief of China's Public Security Bureau, said in 1993.
Since Hong Kong's 1997 handover from British colonial rule, some of Beijing's most ardent critics have found themselves at the wrong end of a triad assault.
Radio host Albert Cheng, publisher Chen Ping and Jimmy Lai, owner of the Apple Daily tabloid, have all survived triad attacks.
In 2014 former Ming Pao newspaper editor Kevin Lau was nearly killed in a machete attack by two men who said they were paid to "teach Lau a lesson".
Two months later -- as a wave of pro-democracy protests swept through Hong Kong -- a group of men descended on demonstrators in Mongkok.
Many of those later arrested had links with prominent triad gangs Wo Shing Wo and 14K.
Police sources told the South China Morning Post they believed members from the same gangs were involved in Sunday's violence.
- 'Plausible deniability' -
Those who study the phenomenon say there is rarely a clear evidence chain.
"Governments outsource violence to third-party agents for plausible deniability," Lynette Ong, a University of Toronto academic who has studied the patron-client relationship between gangsters and local governments in China, told AFP.
The violence at Yuen Long, she said, "could be part of organised crime groups, or simply people who are willing to work by using their muscle power".
J Michael Cole, a Taiwan-based academic who has studied the island triads, said crime groups were "long suspected of acting as proxies and enforcers for the CCP", referring to the Chinese Communist Party.
He noted the most ardently pro-Beijing political party in Taiwan was founded by Chang An-lo, a former head of the Bamboo Union triads who goes by his gangster name "White Wolf".
Chang spent years in mainland China to avoid warrants for his arrest in Taiwan.
Cole said Beijing and organised crime had "a symbiotic relationship which benefits the CCP politically while allowing triads to make fortunes".
Steve Vickers, who hunted triads as the then-head of Hong Kong colonial police's Criminal Intelligence Bureau, said the authorities had been more lax since the 1997 handover.
"Triad societies have always been a problem but one that was at least well contained during the British Administration," said Vickers, who has since set up a risk consultancy.
"In the period since the handover, the Hong Kong authorities have not cracked down on them to the same extent."
Political analyst Dixon Sing said the Yuen Long attacks were designed "to inspire fear" and to deter people joining future protests.
But that, he said, "has created a large backlash and it may backfire".
Anti-government protesters are now threatening to hold a rally this weekend in Yuen Long.
And Hong Kong is bracing for what could be a seventh consecutive week of political violence.