These machinations have helped reveal the city's often hidden underbelly. Anti-triad investigators say a series of violent incidents - one of which led to a traffic policeman shooting dead a knife-wielding thug in May - stems from the refusal of two leaders elected in 2009, nicknamed Fei Wai and Sam Chuen, to stand down in 2011.

The men who want to take over - known as Chi Fung and Dai Ma - took exception to the pair clinging on to power. In the latest attack, Fei Wai and his henchmen were set upon by a group of around 30 rivals in Ashley Road, Tsim Sha Tsui, on the third day of the Lunar New Year.

Police responded five days later by raiding an entertainment venue controlled by the triad group and arresting 15 mainland prostitutes.

The operation, led by the organised crime and triad bureau and Kowloon West regional anti-triad squad, was the force's way of turning up the heat on the gang ahead of the election.

"We want to send a message to the triad that police are monitoring them closely and they will pay the price if they cause any trouble," a senior officer said.

The Wo On Lok's business is crime - extortion, loan sharking, drug trafficking and the control of nightclubs, mahjong dens and massage parlours. According to police, its turf extends across Yau Ma Tei, Tsim Sha Tsui, Mong Kok and Sham Shui Po.

Police sources say triad gangs maintain some of their traditional rituals, such as the election of new leaders, all of which are illegal. Triad membership, or even claiming to be a member, is a crime under Hong Kong law, but investigators say it is a notoriously difficult offence to prove.

"It doesn't matter how many triad tattoos they have - it's not that straightforward," a source said. "Unless you raid an initiation ceremony or the individual actually admits to being a triad member, it's very hard to prove.

"Costly, long-running undercover operations often don't bring the returns required and often witnesses are hard to come by for obvious reasons."

In the May incident, only one of 16 people arrested was charged. Late last year, he was jailed for 3½ years for wounding.

The Wo On Lok, also known as Shui Fong, was founded by a group of workers in a soft-drinks factory in Sham Shui Po more than 70 years ago.

Police figures show reports of triad-related crimes rose 6 per cent to 2,340 last year from 2011.

The Triad Rivals

The Sun Yee On is probably the city's biggest triad society in terms of membership and the most influential, best organised and wealthiest.

Founded decades ago by Chiu Chow and Hokkien immigrants from northeastern Guangdong who speak the Fujian dialect, the society has as its traditional stronghold Tsim Sha Tsui East, but its influence extends to Wan Chai, Tuen Mun and Tseung Kwan O. Its activities include drug trafficking, loan sharking, extortion and smuggling.

Its main rival, the 14K triad, was formed by a Kuomintang general in 1945 to fight the Communists. It is active in West Kowloon, Yuen Long, Kwun Tong and Eastern District, and engages in drug trafficking, prostitution, extortion and pirated goods.

The Wo Shing Wo, the first of the "Wo" family of triads, is indigenous to Hong Kong. It was first active in Tsuen Wan but its influence has spread to West Kowloon, Sheung Shui and Fanling. It controls red-minibus routes and is involved in underground gambling dens, drug trafficking and pirated goods.

Other triads from this group include the Wo Hop To. Active in Western District and Aberdeen, it engages in extortion and loan sharking and controls red-minibus routes.