Klitschko said that he had quit talks after Yanukovych made an unconditional demand that the central square in Kiev be cleared.

"Unfortunately, I bring nothing good from the talks," he said. "The government must immediately withdraw troops and put an end to the bloody conflict, because people are dying. I told Yanukovych this," Klitschko said. "How can we hold talks while blood is being shed?"

It remained unclear on Wednesday morning whether the government was still seeking an agreement with other factions of the opposition. Later on Wednesday morning, Yanukovych said he would talk "differently" to opposition leaders if they distanced themselves from the more radical elements of the protest.

The meeting between Klitschko and Yanukovych took place at the end of the bloodiest day since the unrest began at the end of November last year. At least 25 people - 16 demonstrators and nine police - were reported dead by authorities. Many were killed by gunshots, and hundreds more people were reported to have been injured.

Police moved in on the main site of protest, Kyiv's Independence Square, late on Tuesday and into the early hours of Wednesday, using stun grenades, tear gas and rubber bullets to make inroads.

A ribbon of fire burned throughout the night on Independence Square , also known as the Maidan, as protesters - some armed with clubs and wearing body armor - stood their ground. Tents and tyres were set alight to form a burning barricade across the square that separated police and protesters.

Ukraine's state security service had earlier set a deadline for the demonstrators to end disorder or face "tough measures" as the violence turned deadly.

Fire station torched

There were also major disturbances in at least three cities in the western part of the country, where support for Yanukovych is at its lowest. Police said protesters had taken over regional administration headquarters in the cities of Ivano-Frankivsk and Lviv. Meanwhile, local media said activists had set fire to the main police station in the city of Ternopil.

US Vice President Joe Biden late on Tuesday telephoned Ukrainian President Viktor Yanukovych over the violence, urging him to pull back security forces.

"The vice president further underscored the urgency of immediate dialogue with opposition leaders to address protesters' legitimate grievances and to put forward serious proposals for political reform," said a White House statement.

Activists first began to occupy Independence Square after Yanukovych opted out of an integration deal with the European Union in November, ostensibly under Russian pressure. The latest violence followed the passing of new laws placing curbs on free speech and introducing tough new anti-protest laws.

rc/jm (AFP, AP, dpa, Reuters)