President Viktor Yanukovych has defiantly refused a chorus of calls from global leaders and opposition parties to halt a ferocious assault on anti-Government protesters on the bloodiest day since demonstrations broke out in November.

At least 25 people are dead and large parts of Kiev's occupied Independence Square are burning after a bloody escalation of Ukraine's three-month political crisis.

Yanukovych blamed opposition leaders for the latest violence in Kiev, urging them to "distance themselves from radical forces".

In an address to the nation he said anti-government protesters had "crossed the limits" by taking to the streets to try to oust him from power.


"The leaders of the opposition have disregarded the principle of democracy according to which we obtain power through elections and not on the street ... they have crossed the limits by calling for people to take up arms," he said, adding those responsible would face the law.

The scene in what is known locally as the Maidan has been described as apocalyptic, with explosions echoing around central Kiev and the iconic square effectively turned into a war zone.

Helmeted protesters hurling rocks, fireworks, broken paving stones and Molotov cocktails fought back against black-armoured riot squads targeting them with stun grenades, water cannon and - some said - live rounds.

Smoke from burning tyres billowed up into the cold night air, while laser lights, flags and the glare of several fires from burning tents flickered over the crowd.

Even the opposition headquarters, the imposing Trade Union House, went up in flames, and a frantic effort was mounted to save people inside. Many jumped from windows and were caught by protesters using tents.

"If we hold on until morning, we have a chance to make it out," one protester said.

Police continued to make small gains over night, pushing further into the square at weak points and forcing protesters to torch tents, equipment and other detritus in an attempt to extend their fire barricade.

But the offensive that began the day seemed to have been slowed, protesters said, by the destruction by firebombs of a police APC and a water cannon truck.

Protesters had been preparing for an assault on their camp in independence square for over a month, and the street fighting skills they learned during the outbreak of violence in January was on full display.

Volunteers ripped up vast swathes of pavement to provide cobble stones for the front line, while human chains maintained a steady supply of tyres, wood and other fuel to the barricades.

Deprived of the hospital that ran for three months in the House of the Unions, medics treated walking wounded in the open air.

In a pancake cafe at the back of the square medics in blue scrubs performed surgery on injured protesters sprawled across table-tops, painstakingly removing the shrapnel from the bloodied back legs of two patients. Both patients were wounded worse on the back legs and buttocks - symptomatic of turning away from an incoming stun grenade before it explodes.

Yet amidst all the chaos, the daily life of Maidan carried on almost as normal. Above the din of explosions speakers continued to address a crowd of thousands as they have for three months, leading them in prayers, renditions of the national anthem, and exhortations to "take not one step back".

But with Yanukovych declaring the violence an "armed rebellion", and activists in western regions reportedly raiding police arms depots, many here are now whispering two words that were unthinkable even a month ago: civil war.

The entrenched position of the President was clear earlier, when a meeting between Yanukovych and opposition leaders failed to find a solution to the crisis. "Yanukovych said that there is only one option ... to clear Maidan and that everyone has to go home," opposition leader Vitali Klitschko said.

Kiev was essentially in lockdown as authorities halted the city's metro system and limited road traffic coming into the capital.

Police said seven officers died from gunshot wounds, while authorities and medics counted nine civilian deaths. Another two civilians were found dead, but their bodies showed no exterior signs of violence, making it unclear how they had died.

At least 241 people were injured, including dozens of police officers, some with serious wounds.

As the turmoil intensified, US Vice-President Joe Biden phoned Yanukovych to express his concern and urge him to "pull back Government forces and to exercise maximum restraint", said a White House statement.

EU foreign policy chief Catherine Ashton said she was "deeply worried about the grave new escalation".

UN chief Ban Ki Moon called for restraint and dialogue, while Nato head Anders Fogh Rasmussen urged "all parties to refrain from violence and to urgently resume dialogue, including through the parliamentary process".

German Foreign Minister Frank-Walter Steinmeier warned the violence could lead to EU sanctions against those responsible.

Sweden's foreign minister Carl Bildt tweeted: "Only person who can now stop catastrophe in Ukraine is President Yanukovich."

However a top Russian MP said Ukraine - which is divided between a pro-EU half and a pro-Russian half - was on the brink of a civil war that he claimed had been inflamed by the West.

"I consider that a significant amount of responsibility for this falls on the West and Western politicians, who are constantly putting pressure on the Ukrainian authorities," Alexei Pushkov told Interfax news agency.

The violence erupted after the crisis appeared to have abated in recent days, with both sides making concessions which saw protesters vacate Kiev city hall after being granted an amnesty deal.

Thousands of anti-Yanukovych protesters clashed with police outside Parliament as they rallied for MPs to strip the President of a raft of powers. Running street battles broke out and protesters took back control of city hall and attacked Yanukovych's party headquarters.

After threatening "grave actions" if the unrest did not cease, government sent in the riot police in the evening.

Police warned women and children through loudspeakers to leave the square as they began their assault.

But some 20,000 people, many of them wearing makeshift body protection and wielding iron bars and bats, remained to defy the riot squads.

"This is a small island of freedom," former boxing champion Klitschko said, declaring the protesters were "not going anywhere".