Hong Kong authorities said police allegedly involved in the beating of a pro-democracy protester would be removed from their positions after footage of the overnight incident went viral, sparking outrage from some lawmakers and the public.
Police said they arrested 45 protesters in the early hours of Wednesday, using pepper spray on those who resisted, as they cleared a major road in the Chinese-controlled city that had been barricaded by pro-democracy demonstrators with concrete slabs.
Several officers appeared to beat and kick one protester for several minutes after dragging him to a dark corner next to the protest site in footage aired by television broadcaster TVB.
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Hong Kong Secretary for Security Lai Tung-kwok told a news conference that police would investigate the suspected use of excessive force. The officers shown in the video would be removed from their positions, Lai added.
The incident threatens to inflame tension in Hong Kong, where more than two weeks of protests over Chinese restrictions on how Hong Kong will choose its next leader in 2017 had been losing steam.
Alan Leong, leader of Hong Kong's pro-democracy Civic Party, identified the person in the video as Ken Tsang Kin-chiu and said he was a member of his party.
Civic Party legislator Dennis Kwok, a lawyer representing Tsang, said police also beat Tsang inside a police station. Tsang had since been taken to hospital, Kwok said.
Photographs showing Tsang with bruising on his face and body, released by democracy activists, sparked anger and condemnation across Hong Kong.
Police, without referring to Tsang, said in a statement they had used minimum force, including pepper spray, to disperse protesters who had gathered illegally overnight.
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The operation was the toughest against largely student protesters in more than a week, and came after demonstrators swarmed into a tunnel on a main four-lane thoroughfare late on Tuesday, halting traffic and chanting for universal suffrage.
"There were so many police. They punched people ... We are peaceful," Danny Chiu, a student in his 20s, told Reuters, breaking down in tears.
The tunnel in the Admiralty district near government headquarters was reopened after police cleared away makeshift barriers made out of concrete slabs.
Read MoreHong Kong police make fresh attempt to clear barricaded roads
Protesters have been demanding full democracy for the former British colony.
They are also calling for Hong Kong's pro-Beijing leader, Leung Chun-ying, to step down.
But their campaign, now into its third week, has caused traffic chaos and drained public support for their actions.
China rules Hong Kong under a "one country, two systems" formula that accords the city a degree of autonomy and freedom not enjoyed in mainland China, with universal suffrage an eventual goal.
Beijing said on Aug. 31 that only candidates that get majority backing from a nominating committee stacked with Beijing loyalists would be able to contest a full city-wide vote to choose Hong Kong's next leader.
China's ruling Communist Party believes it has offered enough concessions to Hong Kong in the past, and would give no ground to the protesters because it wants to avoid setting a precedent for reform on the mainland, sources told Reuters.
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The position was arrived at during a meeting of the new National Security Commission chaired by President Xi Jinping in the first week of October, the sources said.
Main protest camp remains
On Tuesday, police used chainsaws and sledge-hammers to clear blockades on another major road in Admiralty.
But hundreds of protesters then stormed into the nearby tunnel, catching authorities by surprise.
The tunnel on Lung Wo Road, an important east-west artery near the offices of the government and legislature, had been intentionally left open by demonstrators to traffic.
Despite the reopening of the two major roads to ease what police said was traffic congestion, there was no immediate sign the core protest zone outside government headquarters, where hundreds of tents remain pitched on an eight-lane highway, would be cleared. Protesters are also loosely scattered around other parts of the Admiralty district.
Smaller groups remain in the busy shopping district of Causeway Bay and across the harbour in the gritty and densely populated area of Mong Kok.
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Leung said this week there was "zero chance" China's leaders would give in to protesters' demands and change the August decision limiting democracy.
Police, criticised for using tear gas and batons in the first 24 hours of the protests, have adopted a more patient approach, counting on protesters to come under public pressure to clear main arteries. In recent days, police have selectively removed some barriers on the fringes of protest sites.
The police action in the early hours of Wednesday, however, suggests official patience may be wearing thin.
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The number of protesters has fallen off sharply from a peak of about 100,000, but a hardcore group of perhaps several thousand remain.