The first punch to the chest was met with a step forward, the second and third to the face with a raising of the chin. The fourth, then fifth appeared to knock him unconscious. 

But, getting to his feet, and with his hands kept firmly behind his back, he faced down his assailant again, and again, and again, a symbol of what has largely been peaceful opposition against an increasingly authoritarian power. 

The assailant appeared to take exception to one of the many “Lennon Wallsâ€? in Hong Kong, which is in the grip of unprecedented political unrest against the erosion of civil rights in the semi-autonomous region of China. 

The walls, which have started appearing across the city in recent weeks, are plastered with colourful notes protesting an extradition bill declared “deadâ€? by the government but still not officially withdrawn. 

The video was shared earlier this week by Edith Leung, a member of staff at the Democratic Party of Hong Kong, a centre-left party and the third largest in the Legislative Council. 

Aftermath of protesters storming Hong Kong government HQ: In pictures

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It shows a man repeatedly punched by an older man, who later tears down dozens of the notes plastered to a wall in Kowloon Bay.

A number of people watched as he was later led away by police.

“What the hell is happening? This is insane!â€? Ms Leung wrote, before later appealing on behalf of the victim’s uncle for witnesses. 

Ray Chan, a former Hong Kong parliamentarian, tweeted: “In Hong Kong, pro-Cops & pro-China thugs are just around the corner in your neighborhood. The young man showed real courage and wisdom as he defended the Lennon Wall.�

Hong Kong human rights campaigner Johnson Yeung described the video as “painful to watchâ€?. 

“I salute the young person who didn't fight back nor back off from violence. He was hit 13 times by pro-Beijing fellow in the face, and he didn't fight back,â€? he said. 

Young people have been at the forefront of huge rallies in recent weeks against the extradition bill, which would have allowed those suspected of crimes to be sent to mainland China for trial.

Activists have vowed to keep up the pressure until Hong Kong leader Carrie Lam officially withdraws the bill. Many are also demanding her resignation.

Wang Zhimin, the Chinese government's chief representative in Hong Kong, said that the central government in Beijing firmly supported Ms Lam. 

Dismissing calls to exonerate protesters who have been arrested, he said it would be "a blatant challenge to the rule of law in Hong Kong." 

"Regarding the recent series of violent incidents, all Hong Kong people, including those present, have expressed condemnation," he said in a speech to Beijing loyalists in Hong Kong.

The protests against the proposed extradition legislation have given voice to fears that Hong Kong is losing the freedoms guaranteed to it when the former British colony was returned to China in 1997. 

Critics fear suspects would face unfair and politicized trials in mainland China, and that critics of the ruling Communist Party would be targeted.

On 1 July, the 22nd anniversary of Hong Kong's handover from Britain back to China, a peaceful march drew hundreds of thousands of people but was overshadowed by an assault on the territory's legislative building. 

A few hundred demonstrators ransacked the building, spray-painting slogans on the chamber walls, overturning furniture and damaging electronic voting and fire prevention systems. 

Tens of thousands of people joined the most recent rally on Sunday, seeking to carry their protest message to those in the mainland where state-run media have not covered the protests widely.

Additional reporting by Associated Press