Ukrainian police and protesters clashed outside the city hall in Kiev on Wednesday after security forces attempted to retake the building occupied by some 200 opposition activists for over a week, an AFP correspondent said.
Police used truncheons outside the building to beat protestors, who responded with sticks. Meanwhile, demonstrators inside the building sprayed police on the street with freezing water from a fire hose, forcing them to retreat into police buses.
Amid an increasingly tense standoff, thousands of protesters came to picket the city hall where deputies from the nationalist Svoboda (Freedom) movement held negotiations with the security forces over their next course of action.
The police buses drove away from city hall.
Meanwhile activists proudly waved flags of the Svoboda party and Ukraine from the upper floors to show that they remained in control of the building. Those on the ground chanted the main slogans of the protest movement, such as "Out with the gang!".
Earlier security forces stormed the epicentre of mass protests but demonstrators refused to leave.
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 (midnight GMT).
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 an AFP 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.
"Kievans rise up! Only together can we win our right to live in a free country," world boxing champion and leader of the UDAR (Punch) opposition party Vitali Klitshcko told the protesters.
Police at the scene said their aim was not to disperse the entire protest but to free up Kiev's main Khreshchatyk Street -- that passes through the middle of the square -- for traffic.
The leader of the nationalist Svoboda (Freedom) movement Oleg Tyagnybok said that several protesters had been injured and the authorities had detained 11 people.
Yanukovych's decision to scrap key trade and political agreements with the EU and police violence against protesters have plunged the ex-Soviet country into its most acute political crisis since the Orange Revolution.
The police action came just hours after Yanukovych held talks with Ashton and prompted swift personal condemnation from Kerry.
Kerry said Washington "expresses its disgust with the decision of Ukrainian authorities to meet the peaceful protest... with riot police, bulldozers, and batons, rather than with respect for democratic rights and human dignity".
"This response is neither acceptable nor does it befit a democracy," he added.
The EU's delegation in Kiev said it was seeking to contact Ukrainian authorities to prevent the use of violence against citizens.
"The authorities didn't need to act under the cover of night," Ashton said in a statement.
In a sign of Europe's support for the demonstrators, Ashton late Tuesday personally visited the epicentre of the protests on Independence Square, known locally as the Maidan.