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.
Mr Yanukovych declined to sign the pact with EU leaders at a summit in Lithuania yesterday, going back on a pledge to work towards integrating his ex-Soviet republic closer to the European bloc.
He said the cost of upgrading the economy to meet EU standards was too great and that economic dialogue with Russia would be revived.
After the police action, about 200 demonstrators shifted the scene of their protest to St. Michael's cathedral - formerly a 12th century monastery which was destroyed by Soviet authorities in 1937 and rebuilt after independence in 1991.
"We gathered here after riot police beat us and chased us out of the square. It's the only safe place we could go to," Roman Tsaldo, 25, said.
Opposition demonstrators said the riot police stormed into the encampment of protesters without warning.
"Police started to break in from behind. They started to beat girls. The girls ran towards us and we couldn't hold," said protester Andriy.
"People started falling from the stage and we were running down Khreshchatik Street to McDonalds. People didn't resist at all," he added.
"It was absolute savagery. By my count, we are talking of tens of cruelly beaten people perhaps hundreds," an opposition deputy, Andriy Shevchenko, was quoted as saying by the Interfax news agency.
"Ukraine has woken up in a different state after Yanukovych refused to sign in Vilnius. It is no longer Ukraine. It's closer to Belarus," said opposition leader Arseny Yatsenyuk, referring to neighbouring Belarus which has been under one-man rule since 1996.
"Yanukovych has already started training for the (presidential) elections in 2015," said Mr Yatsenyuk, a former economy minister.
"He will carry out the elections with the 'Berkut' (riot police), special units, shields, helmets and batons," he said.
The opposition said the riot police stormed into the encampment of protesters without warning.
"There were no grounds for it. There was no provocation by the crowd. People were singing, warming themselves by campfires," said Mr Shevchenko.
Heavyweight boxing champion turned opposition politician Vitaly Klitschko said: "After the savagery we have seen on Independence Square we must send Yanukovych packing."
"They undermined the agreement (with the EU) so as to untie their hands for outrageous behaviour which would be unthinkable by European standards," said Mr Klitschko, a likely contender for the presidency in 2015.
At least four people were beaten by police earlier on Friday night, including a Reuters cameraman and a Reuters photographer,who was bloodied by blows to the head by police.
Speaking to crowds on Friday night, Mr Klitschko said: "Today they stole our dream, our dream of living in a normal country. The failure to sign the agreement of association is treason."