Kiev police chief Valery Koryak said that police used the teargas after one officer was dragged into the crowd and beaten.
The officer was hospitalised with a head injury, while another three officers were also hurt in the violence.
About 200 opposition demonstrators had gathered outside the Kiev city hall to protest against a court ruling extending the authority of the current city legislature.
The opposition is pushing for a vote to elect a new legislature.
The skirmishes erupted when several opposition members of the national parliament tried to enter the building and were turned away by police.
Source: The Guardian