China's shadowy triads have long been suspected of doing Beijing's dirty work.

Perhaps most commonly associated in the West with over-the-top action sequences in kung-fu films, the term “triad” refers to various sects of criminal gangs which often harbour intense rivalries with each other.

Hong Kong is a hub of triad activity, a legacy of the Communist era purge of organised crime from the mainland.

The ancient gangs have been associated with the suppression of protesters and troublemakers in both Hong Kong and parts of southern China in recent years, where they act as thugs for hire.

Sunday's attacks were carried out in plain view of security cameras, and with a mysterious lack of police presence.

The most prominent triad gangs in Hong Kong are 14K and Sun Yee On, and there have been claims that the Wo Sing Wo group are behind Sunday’s attack.

In 2014, triad thugs riding a motorcycle attacked the outspoken editor of a Hong Kong newspaper editor with a meat cleaver, leaving a six-inch wound on his back. Members of the Shui Fong triad, it has been claimed, carried out the attack in exchange for payments of one million Hong Kong dollars (£100,000) each.

