Protesters braced on Wednesday for a fresh assault by riot police in central Kiev after a day of clashes left at least 25 people dead in the worst violence since the start of Ukraine's three-month political crisis.

The protesters marched to the parliament building in a move to keep up pressure on President Viktor Yanukovich to relinquish some of his presidential powers.

As dawn rose over Kiev's battered city center, protesters hurled paving stones and Molotov cocktails at lines of riot police that had pushed into the heart of the devastated protest camp on Independence Square.

Overnight, security forces rained a volley of tear gas down on thousands of demonstrators as they ramped up attempts to clear the square where protesters have set up a sprawling tent city during three months of protests.

Swathes of the tent encampment have already been destroyed by fire and flames poured out of the windows of a gutted building on the square.

A wall of smoke and flames rose up into the dawn sky as the encampment continued to burn, while lines of police and protesters - both clutching shields and wearing helmets and body armor - faced off in an apocalyptic scene.

The surge in violence, in a country torn between a future allied to the West and to Russia, sparked alarm in Europe and the United States.

But a defiant President Yanukovych rejected calls to halt the ferocious assault on the bloodiest day since protests broke out in November, when he ditched a pact with the European Union in favor of closer ties with former master Russia.

In an address to the nation as clashes raged, he said the opposition had gone too far and accused them of trying to oust him.

"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.

NTJ/BA