HONG KONG (AFP) - Hong Kong police said on Monday they had arrested four people over the killing of an alleged triad boss who was hacked to death outside a public hospital in front of horrified onlookers.

The gruesome murder took place shortly after noon on Sunday when the 30-year-old, surnamed Tse, was struck multiple times with an axe and a meat cleaver by two masked men wearing black, the Standard newspaper said.

One blow nearly disembowelled him, the report said.

"It was horrible... his belly was wide open and his intestines were hanging out," a passer-by told the Standard, which said Tse was a senior member of the city's Wo Shing Wo triad society, citing an undisclosed police source.

Television news footage from Cable News showed blood splattered on the ground at the crime scene, along with a yellow-handled axe, walking stick, smartphone and glasses.

"Police officers sped to the scene and found the man lying unconscious on the ground," said a police statement, adding that Tse sustained multiple wounds. He was rushed to hospital but certified dead an hour-and-a-half later.

The arrested men are aged between 18 and 30, police said.

Tse had just seen a doctor in rural Sheung Shui district and was limping with the help of a walking stick when he was attacked, the Standard reported.

Around 100 people thought to be members of the Wo Shing Wo group were seen burning incense and offering prayers at the scene of the murder late on Sunday, the newspaper said.

Hong Kong's notorious triads have a hand in extortion rackets, prostitution, drugs and copyright piracy - although violence is rarely seen in tourist areas.

In 2011 three Hong Kong triad members were jailed for life for the grisly murder of a crime boss who was rammed by a vehicle and then hacked to death outside a luxury hotel.

Hong Kong police last August said they had arrested nearly 1,200 people in raids on illegal gambling dens and brothels during a month-long crackdown on triad activities.