Ukraine’s President Viktor Yanukovych has blamed opposition leaders for the latest upsurge of violence in Kiev, which has left at least 18 people dead, BBC News reports.
After failed overnight talks with the opposition, Mr Yanukovych urged its leaders to “distance themselves from radical forces”, saying it was “not too late to stop he conflict”.
Riot police are continuing to storm the main anti-government protest camp.
Activists blamed the authorities for the worst violence seen in weeks.
Explosions are taking place, fireworks are being thrown and large fires can been seen on Independence Square – known as the Maidan.
The latest assault began shortly after 04:00 local time, when the police tried to move on the protesters’ tents. Several were set ablaze.
Security forces had given protesters a deadline of 18:00 local time 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 have been resisting, throwing missiles from behind huge 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 Mr Klitschko.
Arseniy Yatsenyuk, who heads the Fatherland party, appealed to President Yanukovych to “stop the bloodshed and call a truce”.
Foreign governments have called for an immediate halt of violence, urging political dialogue.
Ukraine’s unrest began in November, when President Yanukovych rejected a landmark association and trade deal with the EU in favour of closer ties with Russia.
But the opposition demands later widened to include what it describes as widespread corruption and abuse of power.
The authorities deny the claim, saying the opposition wants to use the protest to illegitimately seize power.