VISITORS to Hong Kong often head to Mong Kok, Tsim Sha Tsui and Yau Ma Tei for shopping and street food.
But the three areas in West Kowloon also form a so-called "triad triangle" where organised criminal groups have long thrived, with some operating legitimate businesses such as cha chaan teng (tea houses) and nightclubs.
On Friday, that underworld tradition emerged into the glare of the global media, when groups of people, mainly men, descended on protesters who had been staging a pro-democracy sit-in at the busy intersection of Nathan Road and Argyle Road in Mong Kok.
Over the afternoon and night, sustained fist fights broke out, resulting in bloodied heads and other injuries.
Police later arrested 19 people, eight with suspected triad links. More could be on the way, announced the police on Saturday. Further tussles erupted in the area that day.
The attacks, say experts here, highlight the need for more pro-active action to be taken in cracking down on triads.
"There has been less practical focus on their leadership over recent years; waiting for them to commit an overt crime before acting is a recipe for disaster," says Mr Steve Vickers, a former chief of the Criminal Intelligence Bureau during the British colonial era.
"It is unclear who tasked them last night to be active.
"What is clear is that it is time for a crackdown on the triad leadership - especially in Kowloon West," argues Mr Vickers, who now heads a risk consultancy in Hong Kong.
Triads have a long history - and mixed repute - in Hong Kong and China.
Their original incarnation was the Hung Mun, established as a "loyal and righteous society" in the 1800s to overthrow the foreign Qing dynasty. Members were viewed as brave heroes rather than criminals.
But after China became a republic in 1911, it disintegrated into dozens of separate societies, such as 14K and Sun Yee On, engaging in criminal activities. Many members later came over the border into Hong Kong.
Today, Hong Kong has 15 to 20 active triads, according to past reports. Four have 30,000 members each.
And unlike in the days of yore, members now are driven by money, rather than any form of political ideology, says triad researcher Sharon Kwok.
"They are quite politically neutral. They are not pro-government or pro-democracy. They will do something for you - so long as you pay."
This means that the triads in Hong Kong have served paymasters across the political spectrum - sometimes simultaneously.
During the second World War, for instance, they worked with the Japanese occupying forces in organising vice trades such as prostitution, narcotics and gambling, while splitting the revenues.
Later, in the run-up to the 1997 handover, Beijing was striving to maintain public order in Hong Kong and cultivated them as allies. China's Public Security Minister Tao Siju said in 1992 that some triad members were "patriots" and should be respected if they upheld Hong Kong's prosperity.
The late Chinese patriarch Deng Xiaoping made similar remarks in the 1980s and was under the triads' protection when he visited the United States in 1979, criminology academic Lo Tit Wing, citing Mr Tao, wrote in his 2010 article Beyond Social Capital.
At the same time, the triads were also happy to offer their services when pro-democracy activists in Hong Kong were trying to smuggle the Tiananmen student activists out of China following the crackdown in Beijing in 1989.
They had the muscle and the know-how to navigate sea routes to evade police, and were paid handsomely from HK$250,000 (S$41,000) to HK$300,000 for each person rescued, Rev Chu Yiu Ming, an organiser of the operation called Yellow Bird, told The Straits Times in an earlier interview.
Today, Rev Chu is also one of the three leaders of the Occupy Central movement helping to stage the week-long sit-in in parts of Hong Kong.
On whether there were any scruples about working with the local gangs, he said: "We were not so rational in thinking through the moral issues. At that time, whoever could render help, we took it."
Today, the furore is over who are the ones behind the attacks in Mong Kok.
Men rampaged through the protesters' tent canopies and supplies. One of them, when asked by reporters why he was carrying a knife with which he was slashing the items, retorted: "Why am I carrying a knife? I love eating fruits. Fruits from every country. Like durian!"
With thousands of people massing and scuffles breaking out regularly at different parts of the area, the police appeared overwhelmed. Videos going viral on social media show that in some confrontations, the officers took a while to appear.
Local residents have been increasingly frustrated about the protest's disruptions to their daily routines and lives - although Ms Kwok doubts they have the money to hire triad members. The going rate bandied around for one to "cause some nuisance" is about HK$1,000. This is more than the HK$400 that one gets for showing up at a triad negotiation.
Businesses doing the hiring is a possibility, she thinks. So too, pro-government supporters.
Protest organisers and pan-democrat legislators meanwhile are pointing fingers at the government. Some accuse the police of sitting on their hands and abetting the attackers. Others, such as the lawmakers, go further, accusing the government of colluding with the triads.
Civic Party leader Alan Leong suggested that Chief Executive Leung Chun Ying could be the mastermind, reported the South China Morning Post. "I have strong suspicions that he has directed, he is the director behind what happened yesterday."
There were more colourful comments, including by independent pan-democrat Joseph Lee Kok Long who declared: "Hong Kong has been taken over by triad gangs! It is anarchy!"
The tumultuous developments led to the protest organisers suspending talks with the government - with no end to the week-long impasse in sight.
Such accusations against Mr Leung and the establishment are not without precedent.
In 2012, a scandal broke when it emerged that aides of Mr Leung, then campaigning in the CE race, tried to seek support for him at a dinner organised by Heung Yee Kuk, the rural council in the New Territories. Also at the dinner was a former Wo Shing Wo triad leader.
Local media have also reported of claims that triad members were glimpsed receiving money at rallies in support of Mr Leung.
This year, a cleaver attack on former Mingpao editor Kevin Lau fomented suspicions that Beijing was behind it. Nine men, allegedly triad members, were arrested but the mastermind has yet to be nabbed.
In strong remarks on Saturday, the Secretary for Security Lai Tung Kwok flatly denied any suggestions that the police were working with the triads.
"These allegations are totally untrue, unfounded and beyond belief.
"They are also highly unreasonable and extremely unfair to the police officers who faithfully carried out their duty."
A government official dismisses the charges as "conspiracy theories".
But, he adds, police morale is at a low - which could mean that a few officers may not have been as quick as they otherwise would have been in reacting to the fights.
"The police officers are also human beings," he says. "They have been the subject of abuse, and they have been humiliated when food supplies to them were obstructed by the demonstrators. And many of them are angry.
"So one or two, or some of them, may not be as quick or be trying their best as they otherwise would. I would not rule out that possibility, because they are human beings."
Mr Lai stressed that anti-triad work is a top priority of the police force. A police spokesman has told The Straits Times that Hong Kong has attained "notable successes in combating triads". Last year, 2,035 triad-related crimes were reported, comprising 2.8 per cent of all crimes. This is down from 4 to 5 per cent before 1997.
But, warn experts, this does not mean that triads no longer flourish in Hong Kong. Rather, they have become more adept at taking cover under legitimate businesses. Another difficulty is that the identification of triad members through external qualities is difficult because triad ceremonies closely resemble those of Chinese traditions, such as in the distribution of red packets or the setting up of worship altars.
Trying to pinpoint the masterminds who have hired them for illegal services is also tough, because of the multiple layers in between, according to Ms Kwok.
"There are many middle-men in between," she says.