Ukrainian riot police have stormed the offices of a top opposition party in Kiev and they are poised to dismantle protest camps in front of government buildings.

Ostap Semerak, leader of Fatherland Party, told AP that troops raided the party headquarters and called the situation "insane". Some troops were reportedly walking in its corridors while others were climbing through the windows.

Meanwhile, opposition leaders have pledged to defend Independence Square and the city hall from the assault of riot police which started to dismantle at least two protest sites in nearby streets.

The Interior ministry said no action was being taken from the square itself and claimed police action was needed to remove protest block in front of administrative buildings.

Unrest was sparked by Yanukovich's rejection of a deal granting political and free trade accords with the European Union. Protesters saw the hand of Russia's President Putin behind the move.

President Viktor Yanukovich said that he will discuss the crisis with three ex-presidents to try to solve the standoff. He said he supported the idea of an "all-nation round table".

EU foreign policy chief Catherine Ashton is heading to Ukraine "to support a way out of the political crisis".

The protesters, who have set up makeshift camp sites in the square with log fires and soup kitchens, have given Yanukovich 48 hours to resign. Three central metro stations were shut down over apparent bomb threats.

Between 100,000 and 500,000 rallied in the capital at the height of the protests and in defiance of a ban on public demos.