Video footage showing a group of Hong Kong police officers beating a pro-democracy protester has galvanised the city, ratcheting up tensions in demonstrations that have paralysed large swaths of the city for more than two weeks.

The Hong Kong television station TVB showed about six plainclothes officers in police vests leading the man, later identified as Ken Tsang – a social worker and member of Hong Kong’s pro-democracy Civic party – away from a protest site, his hands bound behind his back. The officers took him to a dark corner behind a nearby building and threw him to the ground. Some kicked and beat him, while others kept watch.

Pictures posted to Facebook showed Tsang in the aftermath of the attack, with cuts and bruises on his face and neck, and circular welts running down his back.

Ken Tsang, a Hong Kong pro-democracy activist, is taken away by police. Photograph: Philippe Lopez/AFP/Getty Images

“This is an apparent abuse of police power that a society as civilised as Hong Kong would definitely not swallow,” Alan Leong, the leader of the Civic party, told CNN. “I would advise the commissioner of police to immediately arrest the six officers involved in that attack.”

Hong Kong’s secretary for security, Lai Tung-kwok, said the officers involved would be temporarily removed from duties, as authorities expressed concern over the clip and promised an investigation.

“The Complaints Against Police Office has already received a relevant complaint and will handle it in accordance with the established procedures in a just and impartial manner,” the Hong Kong information services department said in a statement on Wednesday.

Facebook Twitter Pinterest Ken Tsang shows the extent of his injuries

The incident came as police launched a concerted attempt to disperse the pro-democracy protesters using riot shields, batons and pepper spray, prompting some of the most violent scenes since the demonstrations began more than two weeks ago. Police said they arrested 45 people and that four officers were injured in the violence.

It was the most violent crackdown since the first weekend of the protests, when police tactics – including the use of teargas – backfired by prompting more protesters to come out on to the streets.

The intervention, which included driving protesters from an underpass and removing protest barriers with sledgehammers and chainsaws, is being seen as the start of an operation to clear the protesters, and coincided with the harshest condemnation yet of the protests by Beijing.

Communist party mouthpiece the People Daily said the protests were “doomed to fail” and accused those involved of “exacerbating disorder” through “illegal acts”.

Also on Wednesday, Hong Kong’s most prominent tycoon, Li Ka-shing, broke his silence over the protests to urge the demonstrators to go home. Li, Asia’s richest man, said that if Hong Kong’s rule of law broke down it would be the city’s “greatest sorrow”.

“Since the handover, the ‘one country, two system’ formula has protected Hong Kong’s lifestyle,” Li said, referring to the formula under which the city has been run since its return from British to Chinese rule in 1997.

“I urge everyone not to be agitated. I urge everyone not to let today’s passion become the regret for tomorrow. I earnestly request everyone to return to their families,” Li said in his first public comments on the protests.

Wednesday’s police operation came hours after a large group of demonstrators blocked an underpass in the Admiralty area near government buildings that have been the focus of the protests. Demonstrators appeared to storm the short tunnel in reaction to police attempts over the past two days to chip away at barricades on the edges of the sprawling protest zone.

Hong Kong police officers moved in on pro-democracy protesters who blockaded a road near the city government’s headquarters. Photograph: EyePress/Sipa/Rex

Lai Tung-kwok, the security secretary, defended the crackdown, saying in a statement: “A large number of protesters gathering at Lung Wo Road were found dashing to the carriageway, charging the police cordon and throwing objects. The protesters also snatched police barriers and set up roadblocks with plastic boards and drainage covers. As a result, police officers took swift actions to disperse those assembling unlawfully.”

Joshua Wong, a 17-year-old student leader who has become the face of the protest movement, said the video of the police beating meant trust between police and activists had hit an all-time low.

“The proper action police should take is to bring the protester to the police car, not to take him away and then punch and kick him for four minutes,” he told reporters.

Amnesty International also condemned the “vicious” attack. “It is stomach-churning to think there are Hong Kong police officers that feel they are above the law,” Mabel Au, the director of Amnesty Hong Kong, said in a statement.

Professor Joseph Cheng, from the department of politics at Hong Kong’s City University, said the police operation was being seen as the start of a move to end the demonstrations.

Speaking to BBC Radio 4’s Today programme, he said: “These activities are broadly perceived as preparations for final clearing activities. What happened was the police wanted to removed the barricades and push the protesters to a smaller section of the road for traffic. The protesters tried to resist and tried to move another section. Minor clashes occurred.”

The protesters, who have being calling for the resignation of Hong Kong’s chief executive, Leung Chun-ying, and elections free of interference from Beijing, have posed an unprecedented challenge to the government. At their peak two weeks ago, organisers said as many as 200,000 people thronged the streets for peaceful sit-ins. The numbers involved have since dwindled.

Positions on both sides have been hardening since the government called off negotiations last week, citing the unlikelihood of a constructive outcome given their sharp differences. Leung has said there is “almost zero chance” that China’s government will change its rules for the election.