A man has been set on fire during an argument on the street in Hong Kong, in what local media reports described as an altercation over national identity.

It comes amid chaotic scenes as protesters disrupted the Monday morning commute in the city, and hours after a police officer shot a protester during a Facebook live stream.

Multiple videos posted online of the incident showed an older man dressed in a green T-shirt arguing with a group of younger men and women.

The video shows the man, who is already bleeding, walking away from the group as he shouts: “You are not Chinese”. They respond saying: “We are Hong Kongers”, before a shouted insult sees the man in green return to confront them, according to the South China Morning Post.

As the argument resumes, a person dressed in black and wearing a face mask runs up from behind the group of young people and douses the man in liquid, before setting it on fire with a lighter, video from behind the man in green shows. The assailant then appears to flee the scene.

The incident occurred shortly before 1pm on a footbridge. Officials said the man was in critical condition in a city hospital.

The Global Times, a Chinese state-run English-language newspaper, quoted police as saying the incident took place in Ma On Shan district. It described the young people showed in the video as “rioters”, a loaded term used broadly across Chinese state media to describe pro-democracy protesters.

Elsewhere on Monday, there were widespread reports of tear gas being fired across the city and a 21-year-old demonstrator attempting to disrupt traffic in Sai Wan Ho was shot in the stomach by a police officer.

At a news conference, police said live rounds had been fired as warnings in a number of locations and that one protester had been hit. He was undergoing surgery in hospital, officials said.

Monday’s clashes represented some of the most brutal scenes yet in Hong Kong’s months-long unrest, and were particularly unusual for taking place during daylight hours. Previously, the violence has tended to begin at night as police try to get protesters to disperse.

Hong Kong protesters out in force after months of turmoil: In pictures Show all 32 1 /32 Hong Kong protesters out in force after months of turmoil: In pictures Hong Kong protesters out in force after months of turmoil: In pictures A protester throws a stone into a building at Hong Kong Polytechnic University on 11 November Reuters Hong Kong protesters out in force after months of turmoil: In pictures Police patrol the streets of Hong Kong the day after an officer shot a protester during a protest on 11 November EPA Hong Kong protesters out in force after months of turmoil: In pictures Riot police stand guard during a protest against police brutality in Hong Kong on 27 October EPA Hong Kong protesters out in force after months of turmoil: In pictures Protesters carry traffic cones to build a barricade during anti-government protests in Causeway Bay, Hong Kong on 11 November Reuters Hong Kong protesters out in force after months of turmoil: In pictures Protesters use obstacles and bricks 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turmoil: In pictures A protester pours water on a tear gas canister fired by police during a mass rally on the 70th anniversary of the founding of communist China on 1 October Reuters Hong Kong protesters out in force after months of turmoil: In pictures A protester carries a vandalised Chinese flag through Hong Kong during a mass rally on the 70th anniversary of the founding of communist China on 1 October EPA Hong Kong protesters out in force after months of turmoil: In pictures Protesters carry a banner that denounces the celebration of the 70th anniversary of the founding of communist China on 1 October EPA

The MTR rail service was suspended across several lines, and the operator said a fire was started on a train at Kwai Fong station.