The interior ministry said protesters in the capital Kiev yesterday night captured a policeman who wore plain clothes while on duty.
He was beaten and taken to the Kiev mayor's office building, which is being occupied by demonstrators. Once there, he was "tortured", the ministry said.
"There they took away the policeman's ID and personal belongings and continued to hit him multiple times," the ministry said in a statement.
The policeman was hospitalised with a head injury and concussion, said the ministry, adding that a member of the Svoboda (Freedom) nationalist party, Yuriy Levchenko, was present during the beating but did not intervene.
More From This Section
The opposition has over the past two months been locked in a confrontation with the government of President Viktor Yanukovych.
Several people including protesters and police were shot dead during a recent outbreak of violence in the Ukrainian capital, parts of which have been turned into a battle zone.
The opposition fears the government may use the army to crush the protest movement which was sparked by Yanukovych's decision to scrap a pact with the European Union in November.
The interior ministry said last week the protesters captured three policemen wounding one of them with a knife. The opposition has cast doubt on that claim.
Dmytro Bulatov said his captors had cut off an ear and driven nails through his hands before dumping him in a forest.