Ukrainian riot police used batons and stun grenades to disperse hundreds of pro-EU protesters early this morning.
It came after a night of violence in Kiev following President Viktor Yanukovych's decision not to sign a landmark pact with the EU.
Witnesses said police fired stun grenades at protesters camped on Independence Square before wading in.
Officers then waded in with batons before chasing protesters into side-streets, demonstrators said.
A total of 35 people were detained for resisting police, the interior ministry said.
There were no confirmed figures for how many people may have been hurt though the opposition said there could be as many as a hundred.
Tension had been building in Kiev yesterday when Mr Yanukovych declined to sign the pact with EU leaders at a summit in Lithuania, opting instead for "revived" economic dialogue with Russia,
The tough police tactics set the scene for possibly more confrontation tomorrow when a pro-EU rally has been called by the opposition, after 100,000 turned out a week ago.
Demonstrators in Kiev have clashed with riot police at a rally in the city protesting against the government's scrapping of a partnership deal with the EU.
Protesters vowed they would stay on the streets until an agreement was signed with Brussels.
Tens of thousands of Ukrainians bearing EU flags and chanting "Down with the gang!" denounced President Viktor Yanukovych's U-turn and decision to opt for closer ties with Russia instead.
It is the biggest street demonstration in Kiev since the Orange Revolution nine years ago.
Opposition leaders called on President Yanukovych to go ahead with the signing of a landmark agreement with the EU planned for a summit in Vilnius on Friday.
Prime Minister Mykola Azarov's government stunned European leaders last Thursday by announcing a suspension of preparations for signing the key pact with the EU.
The draft agreement came after years of negotiations but has been ditched in favour of increased trade and economic links with Russia.
Irish politician Pat Cox was one of two European Parliament envoys who spearheaded the complex negotiations.
Protesters in Kiev demanded the government step down.