Twenty-five people have been killed in fighting between anti-government protesters and police in the capital Kiev, the Ukrainian Health Ministry has said in a statement.
The Interior Ministry said nine police officers were among the dead.
The figure comes amid a night of unrest in Kiev.
Amid cries of 'Glory to Ukraine!' and with flaming tyres lighting up the night sky, thousands of riot police armed with stun grenades and water cannons attacked the sprawling protest camp in the centre of Kiev.
The move followed a day of street battles that left at least 18 people dead and hundreds injured.
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
With the boom of exploding stun grenades and fireworks nearly drowning out his words at times, opposition leader Vitali Klitschko urged the 20,000 protesters to defend the camp on 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."
Many heeded his call.
"This looks like a war against one's own people," said Dmytro Shulko, 35, who was heading toward the camp armed with a fire bomb. "But we will defend ourselves."
As police dismantled some of the barricades on the perimeter of the square and tried to push away the protesters, they fought back with rocks, bats and fire bombs.
Against the backdrop of a soaring monument to Ukraine's independence, protesters fed the burning flames with tyres, creating walls of fire to prevent police from advancing.
A large building the protesters had used as a headquarters caught fire and many struggled to get out. Many of the protesters were bleeding.
Speaking over loudspeakers, police urged women and children to leave the square because an "anti-terrorist" operation was under way.
The protesters appeared to sense that Ukraine's political stand-off was reaching a critical turning point. Waving Ukrainian and opposition party flags, they shouted "Glory to Ukraine!" and sang the Ukrainian national anthem.
Shortly before midnight local time, Mr Klitschko headed to President Viktor Yanukovych's office to try to resolve the crisis.
He returned to the square early today without reaching any agreement on ending the violence. H
Mr Klitschko told reporters that he had asked the president to stop the police action to clear the square and prevent further deaths, but Mr Yanukovych's only proposal was that the demonstrators have to go home and stop the protests.
"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 Mr Yanukovych later today.
The violence began yesterday when protesters attacked police lines and set fires outside parliament, accusing Mr Yanukovych of once 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 Mr Yanukovych's government needs to keep Ukraine's ailing economy afloat.
This raised fears among the opposition that Mr 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 Mr 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.
Russia also may have wanted to see Kiev remain calm through the Winter Olympics in Sochi, so as not to distract from President Vladimir Putin's games.
But after the outburst of violence against riot police, Mr Yanukovych's government may have felt it had no choice but to try to restore order.
While Kiev and western Ukraine have risen up against Mr Yanukovych, he remains popular in the Russian-speaking eastern and southern regions, where economic and cultural ties with Russia are strong.
Yesterday's clashes were the first to lead to deaths since January 22, when two protesters were hit with live ammunition and a third died after a fall.
As angry protesters outside parliament hurled stones at police and set trucks blocking their way on fire, riot police retaliated with stun grenades and fired what appeared to be small metal balls, as smoke from burning tires and vehicles billowed over Kiev.
Early this morning, government agencies said 18 people died in the violence, including seven policemen who died from gunshot wounds.
Eleven civilians also died, including three who were shot. A police spokeswoman said 159 police were wounded, including 39 who were shot.
The coordinator for the opposition's medical response team, Oleh Musiy, said more than 400 protesters were injured. He also claimed that about 20 had died, but this could not independently be confirmed.
One of the civilians was found dead after protesters stormed the office of the president's Party of Regions. Police pushed them away, but when firefighters arrived to put out a fire, they discovered the body of an office employee, Kiev's emergency services said.
In Washington, Vice President Joe Biden expressed his "grave concern" in telephone call to Mr Yanukovych, urging him to pull back government forces and exercise maximum restraint.
Germany has refused to back Washington's calls for sanctions against Ukraine's government to pressure it into accepting opposition demands for reforms.
But when central Kiev exploded in violence Tuesday, German foreign minister Frank-Walter Steinmeier said Ukrainian security forces have a "particular responsibility" to de-escalate the situation, adding that the EU might resort to unspecified sanctions against individuals.
"Whoever is responsible for decisions that lead to bloodshed in the centre of Kiev or elsewhere in Ukraine will need to consider that Europe's previous reluctance for personal sanctions must be rethought," he said.
The Russian foreign ministry blamed the West for the escalation of the violence and called on the opposition to work with the government to find a way out of the crisis.
"What is happening is a direct result of the conniving politics of Western politicians and European bodies," the ministry said.
UN Secretary-General Ban Ki-moon expressed shock at the escalating and "unacceptable" violence and called for "the immediate renewal of genuine dialogue leading to rapid results".