Elite Berkut anti-riot police and interior ministry special forces moved against the protestors who had occupied Kiev's Independence Square for a over a week at around 2:00 am. (Agencies)
US Secretary of State John Kerry expressed "disgust" over the crackdown, which came as EU foreign policy chief Catherine Ashton and US Assistant Secretary of State Victoria Nuland were in Kiev for talks with all parties to find a way out of the crisis.
The security forces tore down the barricades that the protestors angry at President Viktor Yanukovych's rejection of an EU pact under Russian pressure had erected around Independence Square.
Thousands of armoured police seized control of part of the square by forming a human chain but the demonstration continued as before over much of the area with more protesters arriving as morning broke.
With their numbers swelled to at least 10,000 after the police action, dozens formed human walls to prevent more riot police from entering the square from adjoining roads, said a correspondent.
Speakers gave addresses from the stage as Ukrainian pop anthems made famous during the 2004 Orange Revolution blared through loudspeakers.
Thousands of anti-riot police had earlier surrounded the square and then entered the area, using their sheer numbers to force the demonstrators away from the road running through the square.
Police said in a statement that they had not used tear gas so far but reserved the right to "react adequately" in case of violations of the law.
Ukrainian opposition leader Arseniy Yatsenyuk called for a millions-strong protest in Kiev after the police actions, predicting that the regime of Yanukovych would fall.
"We will not forgive this. Here there will be millions and his regime is going to collapse," Yatsenyuk, the leader of the party of jailed ex-premier Yulia Tymoshenko, told the protesters on Independence Square.
"Yanukovych spat in the face of the United States and the EU member states," he said as protesters chanted "out with the gang!"
Thousands of protesters had been defying sub-freezing temperatures to remain overnight on the square in protests that started almost three weeks ago.
Elite Berkut anti-riot police and interior ministry special forces moved against the protestors who had occupied Kiev's Independence Square for a over a week at around 2:00 am.