Police in the Ukrainian capital Kiev have launched a fresh attack on anti-government protesters as the death toll in renewed clashes has climbed to 26.
The new attempt to uproot the protest stronghold came as President Yanukovych blamed opposition leaders for the worst violence in months of unrest.
After failed overnight talks, he urged them to distance themselves from radical forces.
The EU says it expects to agree sanctions on those behind "repression".
Russia, however, has accused opposition protesters of trying to engineer a "violent takeover of power".
Police launched their latest assault on Independence Square, also known as the Maidan, shortly after 04:00 local time (02:00 GMT). Several tents were set ablaze, and water cannon was later used.
The BBC's Daniel Sandford said police had taken control of a corner of the square for the first time since December.
The protests began in late November, when President Viktor Yanukovych rejected a landmark association and trade deal with the EU in favour of closer ties with Russia.
Tensions had begun to subside as recently as Monday, as protesters ended their occupation of government buildings in return for an amnesty against prosecution.
But violence erupted outside parliament on Tuesday morning as government supporters blocked opposition attempts to scale back the president's constitutional powers.
Correspondents say it was unclear what sparked the clashes, with each side blaming the other. Protesters accused pro-government agents provocateurs, known as "titushki", of inciting some of the violence while the government said radical Right Sector supporters were involved.
Fighting spread to surrounding streets and police launched a first attack on Independence Square on Tuesday evening.
The number of dead on both sides has risen to 26 and it is feared the death toll could rise further.
- Ten of those killed were police, the interior ministry says. Two were traffic police officers
- At least 14 protesters were among the dead: many were killed in the streets around the parliament
- A journalist working for Russian-language newspaper Vesti, Vyacheslav Veremyi, was pulled from a taxi by masked men and shot
- Several hundred people have been treated for injuries
As police gained ground in the Maidan, stones and petrol bombs were met with tear gas.
The protesters tried to hold their defence lines, burning tyres on the barricades and more anti-government activists were said to be on their way to join the camp.
A trade union building where many protesters had been sheltering was set alight and people could be seen climbing down the walls to escape the flames.
There were reports of unrest breaking out elsewhere in Ukraine, including the western cities of Lviv, Ivano-Frankivsk and Ternopil.
'Island of freedom'
Opposition leaders Vitaly Klitschko and Arseniy Yatsenyuk met President Viktor Yanukovych for late night talks but failed to come to an agreement.
In a statement broadcast on TV news channels on Wednesday morning, President Yanukovych said: "The opposition leaders have disregarded the principle of democracy according to which one obtains power not on the streets or maidans - but through elections."
"They have crossed the line by calling for people to take up arms," he said, warning that those responsible for violence would face the law.
But the president added that there was a "better and more effective way" to solve the crisis - through dialogue and compromise.
"It is not too late to stop the conflict," he said.
Security forces had given the protesters a deadline of 18:00 on Tuesday (16:00 GMT) to leave the square, the scene of a mostly peaceful protest camp since November.
When the deadline expired, riot police advanced with an armoured vehicle, dismantling barricades and firing stun grenades and water cannon.
Protesters resisted, throwing missiles from behind piles of burning tyres.
In speeches from the main stage through the night, protest leaders urged people already on the Maidan to stand firm, and called on Ukrainians elsewhere to come to the square.
"This is an island of freedom and we will defend it," said Vitaly Klitschko, the leader of the Udar (Punch) party.
Mr Yatsenyuk, who heads the Fatherland party, appealed to President Yanukovych to "stop the bloodshed and call a truce".
He had earlier accused the president of blocking attempts to reform the constitution in parliament.
But MPs who support the president said the proposals had not been thoroughly discussed, and that more time was needed.
There has been widespread international alarm at the bloodshed in Kiev.
- European Commission President Jose Manuel Barroso said he expected EU member states urgently to agree "targeted measures" against those responsible for the violence. Foreign ministers will hold an emergency meeting on Thursday
- Russia's foreign ministry blamed the clashes on the "conniving politics of Western politicians and European bodies"
- Swedish Foreign Minister Carl Bildt said Ukraine's president had "blood on his hands"
- The head of Ukraine's Olympic committee in Sochi, Sergei Bubka, appealed to all sides to stop the violence
- US Vice-President Joe Biden urged President Yanukovych by phone to de-escalate the situation, the White House said