Ukraine's key opposition parties have distanced themselves from an incident on Monday in which two men came out of a car that drove up to the city council building in Kryvy Rih, Dnipropetrovsk region, and flung Molotov cocktails toward the building, injuring policemen.

"The front door was partially damaged. Four police officers were hurt, receiving injuries such as a fractured arm, concussions and burns," the city police authority said in a statement.

The attackers were arrested on the spot.

The three key opposition parties - Batkivschyna (Fatherland), UDAR and Svoboda (Freedom) - distanced themselves from the assault.

The three parties "have nothing to do with this fact," the leaders of the parties' Kryvy Rih branches said in a statement. "We condemn this act and qualify it as a provocation designed to discredit peaceful opposition meetings," they said.

Interfax-Ukraine has learned that one of the attackers was a man who in 1999 had thrown a hand grenade at then presidential candidate Natalya Vitrenko and was sent to prison for this but was released before the expiry of his term. Opposition leaders have denied that he is a member of any party.