The footage aired after overnight clashes between police and pro-democracy demonstrators sparked outrage and calls for prosecution from activists and lawmakers.
It shows six plainclothes officers hauling a handcuffed and unarmed demonstrator to a dark corner in a protest-hit public park, and being placed on the ground.
One officer stands over the man and punches him, as three others are seen repeatedly kicking him.

"Police are concerned over the issue and will carry out a just and fair investigation," Hong Kong's Secretary for Security Lai Tung-kwok told reporters.
"The policemen who are involved in the incident have been removed from their current working positions," he said without specifying how many officers were being probed.
Civic Party, one of Hong Kong's pro-democracy political groups, identified the man in the video as one of its members, Ken Tsang.

"The Civic Party legal team is handling the situation," Civic Party lawmaker Claudia Mo said.

Prominent student leader Joshua Wong said people had lost all faith in the police after the brutal incident.

"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.
The violence was among the worst seen during more than two weeks of rallies that have drawn huge crowds calling for Beijing to grant the semi-autonomous city the right to hold free elections.
The protests have been sparked by China's insistence it will vet candidates standing for election as the semi-autonomous city's next leader in 2017, a move protesters deride as 'fake democracy'.

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