According to eyewitnesses some died as a result of single gunshot wounds, typical of sniper fire.
Officials said that one policeman had died and that 67 police had been captured by protesters.
Meanwhile, three EU foreign ministers have held five hours of talks with President Viktor Yanukovych.
Eyewitnesses have told international news agencies that they have counted between 21 and 27 protesters’ bodies after clashes in Kiev.
Video footage has emerged apparently showing snipers firing on demonstrators who had been trying to retake their protest camp in Independence Square.
Officials said more than 20 policemen had also been injured.
Witnesses reported live rounds, petrol bombs and water cannon being used at Independence Square during this morning’s clashes.
An opposition Twitter feed posted an image of 13 bodies in a makeshift mortuary in the lobby of the Hotel Ukraine.
The hotel is serving as the base for all foreign media in Kiev.
Earlier, several dozen protesters were using the lobby as a triage centre for the wounded.
Protesters – some of them armed – asked hotel guests for blankets to use as bandages.
A statement on the presidential website blames the opposition for starting the violence, saying the “calls for a truce and dialogue were nothing but a way of playing for time to mobilise and arm militants from Maidan [Independence Square]”.
Opposition leaders called the violence “an act of provocation” by the authorities.
The foreign ministers of France, Poland and Germany conducted five hours of discussions with Viktor Yanukovych, French Foreign Minister Laurent Fabius tweeted.
Polish Foreign Minister Radoslaw Sikorski tweeted that he would now stay in Kiev to meet opposition leaders to test a “proposed agreement”, although it was not clear what the details of the agreement were.
Other EU foreign ministers, along with EU foreign policy chief Catherine Ashton, have convened at an emergency meeting in Brussels to discuss possible sanctions.
They could include a possible ban on sales of equipment that might be used for internal repression.
Separately, the head of the Kiev city administration resigned from Viktor Yanukovych’s Party of the Regions.
Thursday had been declared a day of mourning for those killed in clashes on Tuesday.
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