Hong Kong police say man set alight after arguing with protesters

Hong Kong police on Monday said a man was set alight following a heated argument with pro-democracy protesters during a day of widespread clashes across the city, as videos of the brutal attack went viral online.

Three videos were posted on messaging channels used by protesters showing a man in a green T-shirt arguing with people on a footbridge.

A masked man dressed in black then throws a liquid over the man and sets him ablaze as crowds scatter and the victim frantically tries to remove his T-shirt.

"In the most shocking incident, some rioters poured flammable liquid onto a person and set him on fire," police spokesman John Tse told reporters at a press conference as the social media videos of the attack were played on a screen.

"The man has been admitted to hospital in critical condition."

The incident took place in the northern area of Ma On Shan, around 20 kilometres (12 miles) from the central business district, he said.

It came during a tense day in Hong Kong, which has been engulfed for more than five months by increasingly violent pro-democracy protests.

Tse said shortly before the attack, masked protesters had trashed the nearby Ma On Shan train station and investigators believe the man "chased the rioters".

"Afterwards he was beaten up and set on fire," Tse said.

A hospital authority spokeswoman said a man with burns had been admitted to Prince of Wales Hospital, the closest facility to Ma On Shan.

The videos have not been verified by AFP, but the footage was quickly picked up by Chinese state media.

The Global Times tabloid uploaded one of the clips to Twitter -- a platform banned on the mainland -- with the caption: "Black-clad rioters in #HongKong set a resident on fire in Ma On Shan on Monday, Hong Kong police confirmed with the Global Times."

"Black-clad rioters" is a phrase commonly used by Chinese media to describe democracy protesters.

What sparked the argument is unclear, but the expletive-laden conversation shows the man in green criticising people sympathetic to Hong Kong's democracy movement, and his opponents berating him back.

At one point the man in green can be heard saying "none of you are Chinese".

One of his opponents shouts "Go back to the Greater Bay Area", the name for a nearby region of the Chinese mainland across the border from Hong Kong.