Police said they arrested 45 protesters in the early hours, using pepper spray on those who resisted, as they cleared a main road in the Chinese-controlled city that had been barricaded by pro-democracy demonstrators with concrete slabs.
Several officers appeared to beat and kick a handcuffed protester for several minutes after dragging him to a dark corner next to the protest site in footage aired by television broadcaster TVB.
Hong Kong Secretary for Security Lai Tung-kwok told a news conference police would investigate the suspected use of excessive force. The officers shown in the video would be temporarily removed from their positions, Lai added.
Outrage over the beating could galvanize support for the democracy movement in the former British colony where more than two weeks of protests over Chinese restrictions on how it chooses its next leader in 2017 had dwindled from around 100,000 at their peak to a few hundred.
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 the party, Reuters reported.
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.
Tsang is also a social worker. The Hong Kong Social Workers' Association said it planned to march to police HQ in the evening to protest.
Photographs showing Tsang with bruising on his face and body, released by democracy activists, sparked anger and condemnation. Human rights group Amnesty International said the police involved in what appeared to be a "vicious attack against a detained man" should face justice.
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.