At least 25 people have been killed and more than 240 injured during clashes between police and protesters in the Ukrainian capital, health officials said.
The confirmed death toll increased the day after thousands of riot police armed with stun grenades and water cannons attacked a sprawling protest camp in the centre of Kiev, after a day of street battles.
President Viktor Yanukovych blamed opposition leaders, saying he had already made several attempts to compromise but that opponents "crossed a line when they called people to arms".
Yanukovych said opposition leaders had to "draw a boundary between themselves and radical forces", or else "acknowledge that they are supporting radicals".
Earlier, opposition leader Vitali Klitschko urged the 20,000 protesters to defend the camp in Independence Square that has been the heart of the protests.
"We will not go anywhere from here," the former heavyweight boxing champion, told the crowd, speaking from a stage in the square as tents and tyres burned around him, releasing huge plumes of smoke.
"This is an island of freedom and we will defend it."
The violence yesterday was the deadliest in nearly three months of anti-government protests that have paralysed Ukraine's capital in a struggle over the identity of a nation divided in loyalties between Russia and the West, and the worst in the country's post-Soviet history
As police dismantled some of the barricades on the perimeter of the square and tried to push away protesters, the demonstrators fought back with rocks, bats and fire bombs.
Against the backdrop of a monument to Ukraine's independence, protesters fed the flames with tyres, creating walls of fire to prevent police from advancing.
Speaking over loudspeakers, police urged women and children to leave the square because an "anti-terrorist" operation was under way.
Shortly before midnight local time, Mr Klitschko headed to Yanukovych's office to try to resolve the crisis.
He returned to the square without reaching any agreement on ending the violence.
Mr Klitschko told reporters that he had asked the president to stop the police action to clear the square and prevent further deaths, but Yanukovych's only proposal was that the demonstrators stop the protests and go home.
"I am very unhappy because there was no discussion," Mr Klitschko said. "They don't want to listen."
Still, Mr Klitschko urged the protesters and police to stop the escalation of violence. He said opposition leader Arseniy Yatsenyuk was trying to arrange for more negotiations with Yanukovych.
The violence began yesterday when protesters attacked police lines and set fires outside parliament, accusing Yanukovych of again ignoring their demands.
Parliament, dominated by his supporters, was stalling on taking up a constitutional reform to limit presidential powers.
Tensions had soared after Russia said on Monday that it was ready to resume providing the loans that Yanukovych's government needs to keep Ukraine's ailing economy afloat.
This raised fears among the opposition that Yanukovych had made a deal with Moscow to stand firm against the protesters and would choose a Russian-leaning loyalist to be his new prime minister.
The protests began in late November after Yanukovych turned away from a long-anticipated deal with the EU in exchange for a 15 billion US dollar (£9 billion) bailout from Russia.
Until Monday, the government and the opposition had appeared to be making some progress toward resolving the political crisis peacefully.
In exchange for the release of scores of jailed activists, protesters on Sunday vacated a government building that they had occupied since December 1.