Hong Kong police said on Monday they are investigating reports that a man was set alight following a heated argument on a day of widespread protests across the city, as videos of the purported attack surfaced 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, reportedly in the northern district of Ma On Shan.
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.
A spokesperson for the Hospital Authority said a man with burns had been admitted to Prince Wales Hospital, the closest facility to Ma On Shan.
"He's in critical condition," the spokesperson said.
A police spokesperson told AFP that they had received a report on the incident and were investigating.
The videos have not been verified by AFP, but the footage was picked up by Chinese state media.
The Global Times tabloid uploaded one of the videos to Twitter, a social media 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 pro-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.