Attacks by masked men on Hong Kong's pro-democracy protesters have shone an uncomfortable spotlight on the city's pervasive and shadowy "triad" crime gangs, renewing accusations they are working in cahoots with the government.

Hong Kong may be a city of gleaming skyscrapers that prides itself on being a vital regional financial hub renowned for its legal system, thriving port and adherence to international standards.

But the former British colony has never been able to shake off a darker side to its reputation as a hotbed for organised Chinese criminal networks steeped in murky traditions and violent histories.

A recent spate of attacks against protesters calling for unfettered elections has raised concerns among democracy activists that the city's triad gangs -- who have largely remained aloof from the dirty world of politics -- are involving themselves in the ongoing debate over Hong Kong's future.

The protesters have paralysed parts of the city for more than two weeks, erecting barricades and occupying main thoroughfares in their bid to persuade Beijing to allow full democracy in the city.

On Monday, dozens of masked men launched a brazen assault on demonstrators, clashing in broad daylight with the unarmed, peaceful activists in chaotic scenes before a heavy media presence in the city centre.

Lawmakers and protest leaders were incensed that the men made their move shortly after police had carried out dawn operations to remove peripheral barricades in Admiralty district, one of the main protest sites.

"It seems that the police have duly removed some of the barricades to make way for the suspected triads to get through to the peaceful protesters," pro-democracy lawmaker Claudia Mo told AFP.

- Strong denial -

The police and the government were forced earlier this month to deny allegations that they were working with criminals after masked thugs attacked protesters at another demonstration camp in Mongkok, a working-class district known for its triad gangs.

Police said eight of the men they arrested after that incident had suspected triad links.

The city's security chief Lai Tung-kwok angrily denied the allegations after the Mongkok clashes saying they were "made up and very excessive", while police have rejected any suggestion of colluding with gangs.

Whether deliberate or not, witnesses at the clashes said the thugs appeared to be siding with the government -- reinvigorating accusations that the city authorities are either encouraging the attacks, or not doing enough to rein them in.

"It seems the government is indulging people to use force if not contracting out the use of force," pro-democracy lawmaker Albert Ho told AFP after Monday's clashes at the barricades.

"This is one of the tactics used by the Communists in mainland China from time to time. They use triads or pro-government mobs to try to attack you so the government will not have to assume responsibility."

Similar accusations were made in Mongkok earlier this month when a small contingent of police officers were seen making little attempt to halt violent attacks on pro-democracy protesters during clashes there.

However Wang Peng, a Hong Kong University professor specialising in organised crime, questioned whether the authorities would actively cooperate with criminals.

"Employing triad groups as enforcers for the Hong Kong and mainland governments will destroy their legitimacy," he said.

- Money and influence -

Hong Kong's notorious triad gangs have traditionally been involved in drug-running, prostitution and extortion, operations that increasingly operate alongside more respectable ventures in property and the finance industry -- including links with a number of publically listed companies, experts say.

The three largest gangs most commonly cited in local media are Wo Shing Wo, 14K and Sun Yee On, who are active in Hong Kong, southern China and further overseas. There are also multiple smaller triad networks and affiliated gangs.

In March, two triad-connected suspects were arrested for a brutal attack on Kevin Lau, a former editor of the liberal Ming Pao newspaper.

The assault came at a time of rising concern in the city over press freedom, with journalists and newspapers critical of Beijing increasingly targeted.

But while democracy activists accuse triad thugs of involving themselves in politics, others say organised crime groups may have a more base reason for attacking protester barricades -- money.

Mongkok is an area with deep triad connections -- known for its massage parlours, brothels and protection rackets -- and local business owners have complained about falling revenue since the protests broke out.

"Patriotic Chinese businessmen who have links with the Chinese government and want to show their support... may hire triad members to threaten students," said Hong Kong University's Wang.

"Hong Kong businessmen whose businesses are affected by Occupy may have also hired triad members," Wang added.

He said that Hong Kong's triad gangs -- who were prolific during British rule in the 1960s and 1970s, when police corruption was rampant -- had refocused much of their operations to the mainland after the territory was handed back to China by Britain seventeen years ago.

"Since 1997 the triads have transferred their focus from Hong Kong to mainland China," he said, adding a lot of their business is also in the entertainment field.

Police have made periodic sweeps against triad operations, often racking up lengthy arrest sheets.

In a series of raids on night clubs and massage parlours in 2013 police arrested more than 1,800 people, officials figures show. Another 1,200 arrests were made in an anti-triad operation in 2012.

But the capture and incarceration of major triad leaders -- known as "Dragonheads" -- is rare.