The Hong Kong police department is investigating reports that officers used excessive force against pro-democracy protesters.
Local TV showed images of officers apparently beating a handcuffed protester on Wednesday in some of the worst clashes since the protests began.
Hong Kong's security chief said the officers had been "temporarily removed from their current duties".
The incident occurred as police cleared an underpass near government buildings.
The police advance came when protesters blockaded the underpass after being cleared out of other areas of the city on Tuesday.

Overnight police used pepper spray and batons to remove protesters from Lung Wo Road which they said earlier had to be cleared as it was a major thoroughfare. They also arrested 45 people for "unlawful assembly".
Local TV network TVB aired footage that appeared to show a group of plainclothes policeman dragging a handcuffed and unarmed protester and placing him on the ground.
They then assault him, kicking him repeatedly.
The man was named as Ken Tsang, a social worker and member of the opposition Civic Party. He was later taken to hospital.
Secretary for Security Lai Tung-kwok said there was "concern" over a video clip "showing police officers who used inappropriate force against an arrested person".
He said the officers seen on the video would be removed from their current duties and that an investigation would be carried out.

The footage shot by broadcaster TVB has been widely shared on social media.

Accusations of police using excessive force were made when authorities fired tear gas as the protests first erupted in late September. But this incident, which took place at around 03:00 on Wednesday (19:00 GMT Tuesday), was different.
The demonstrator, Ken Tsang, a social worker, had already been detained and no longer posed any threat to law enforcement.
Hong Kong's police force has for years prided itself on its professionalism, political neutrality and experience with crowd control. Now, citizens are asking, why did officers appear to behave with impunity toward a protester who had been subdued?

Bron:BBC
Eigen mening: Ik vrees dat dit alleen maar meer protesten gaat uitlokken en de politie in Hong Kong,zeggen dat ze een reputatie hebben voor professioneel om te gaan met protesten en neutraal te zijn. Dit is moeilijk te geloven als de regering daar de sociale media zoveel mogelijk probeert te beperken.Zo dat zo weinig mogelijk aan het licht komt volgens mij is dit nog maar het tipje van de ijsberd.