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.

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 the conflict".

Riot police are continuing to storm the main anti-government protest camp.

Activists blamed the authorities for the worst violence in months of unrest.

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, and water cannon was later used.

But the protesters appear to have been able to hold their defence lines, burning tyres on the barricades.

Thousands of policemen have been surrounding the Maidan since Tuesday, although reports saying that many anti-government activists are heading to join the camp.

'Island of freedom'

In a statement 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".

Protesting have been defending Maidan barricades from repeated attacks by the riot police

"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 have been resisting, 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 party.

Arseniy Yatsenyuk, who heads the Fatherland party, appealed to President Yanukovych to "stop the bloodshed and call a truce".

At least 18 people - including seven police officers and a journalist - have died, and the death toll is expected to rise further.

Hundreds of people have been injured.

Earlier on Tuesday, police blocked protesters from marching on parliament, where MPs had been due to debate proposed changes to the constitution which would have reduced the powers of the president.

The debate did not take place. Mr Yatsenyuk said President Yanukovych was blocking the reforms and that his allies "show no desire whatsoever to end the political crisis".

But MPs who support the president said the proposals had not been thoroughly discussed, and that more time was needed.

Some protesters outside parliament ripped up cobblestones to throw at police. Police fired stun and smoke grenades, and rubber bullets. Correspondents say it was unclear what sparked the latest violence, with each side blaming the other.

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.