Klitschko said on Monday he had asked the protesters to leave, having visited them overnight, and called for provocations to be avoided.

"They want to stay but I'll try to convince them otherwise," said Klitschko, speaking in the capital, Kyiv.

Radical activists from the group Spilna Sprava, meaning "The Right Deed," took the building late on Sunday, smashing windows and erecting barricades with sandbags and snow.

In response, Justice Minister Lukash told Ukrainian television she would call for emergency measures to be imposed if the activists did not leave.

Lukash said she would approach Ukraine's national security council with "a demand to discuss imposing a state of emergency in this country."

She added that negotiations between the opposition and government – in which she herself has been involved - were being placed in jeopardy.

"I will be forced to ask the president of Ukraine to stop the talks if the building is not freed immediately and negotiators are not given a chance to find a peaceful solution to the conflict," Lukash told the Inter channel.

Opposition leaders a power sharing proposal from President Viktor Yanukovych did not go far enough, demanding snap elections.

'Poisoned offer'

Yanukovych on Saturday offered the opposition posts in government including that of prime minister. He proposed sharing leadership by installing Fatherland party chief Arseniy Yatsenyuk as prime minister and UDAR (Punch) leader Klitschko as deputy prime minister.

Klitschko condemned the proposal as "poisoned" in an interview with German newspaper Bild am Sonntag. "This was a poisoned offer by Yanukovych designed to split our opposition movement," he was quoted as saying.

Protests have spread from the capital, with activists laying siege to local government offices in four Ukrainian cities on Sunday. Police were reported to have used batons and stun grenades to break up the rally in the city Zaporizhya, causing injuries.

Zaporizhya and the eastern cities of Dniepropetrovsk and Sumy - where buildings were also occupied - lie in east of the country, which that has traditionally been sympathetic to Yanukovych.

Ukraine was plunged into a two-month-long crisis after the president refused to sign an association deal with the EU late last November. Last week, tensions were intensified with the controversial introduction of new anti-protest laws.

rc/kms (AFP, dpa, Reuters)