Dozens of Interior Ministry troops and riot police moved in columns into central Kyiv as protesters remained defiant overnight. Helped by police who had cleared the surrounding area of protesters, city employees started dismantling the barricades demonstrators had erected around the Cabinet headquarters.

"We are now going to defend our Maidan," opposition leader Arseniy Yatsenyuk said ahead of the assault, referring to Independence Square, the main protest venue, by its Ukrainian name.

Earlier, riot police took up position outside of Kyiv's city hall, which protesters had occupied for over a week. The move came in the hours following a mass protest attended by several hundred thousand people calling for the ouster of President Viktor Yanukovych. Demonstrations began over a week ago, when Ukrainians took to the streets in angry protest of the president's decision to abandon planned political and free-trade agreements with the European Union.

It was not known how many demonstrators remained on Kyiv's Independence Square by the time riot police arrived on Monday. Previous estimates regarding the total number of protesters have varied, with the opposition putting it at 500,000 people. Separate observers put the figure close to 300,000.

'Images are insane'

Ostap Semerak, a member of jailed opposition figure Yulia Tymoshenko's Fatherland Party, told the AP news agency that troops had broken into the party's offices on Monday evening, some climbing in through its windows. Fatherland represents the largest opposition grouping in Ukraine's parliament.

"They are storming us," Semerak told AP by telephone. "The images are insane."

The troops left after confiscating some computer equipment, he said. News agency reports confirmed broken glass and smashed computers in the offices.

Party member Marina Soroka said the troops also surrounded and blockaded several opposition-minded Ukrainian media outlets, making their and other websites inaccessible.

Tymoshenko, a longstanding foe of President Yanukovych's and a co-leader of the 2004 pro-democracy Orange Revolution, was sentenced by a court to seven years in prison on abuse of power charges in 2011. Critics call Tymoshenko's conviction a case of political revenge.

Biden weighs in

US Vice President Joe Biden called Yanukovych on Monday to express concern about the situation in Ukraine and worries about the potential for violence between the government and protesters. Biden said violence had no place in a democratic society and stressed US support for Ukrainian ties to Europe.

"The Vice President underscored the need to immediately de-escalate the situation and begin a dialogue with opposition leaders on developing a consensus way forward for Ukraine," the White House announced in a statement.

On Monday, Yanukovych announced that he would meet former Ukrainian presidents Tuesday in a bid to find a way out of the crisis and that he also backed the idea of roundtable talks with the opposition. With international concern growing about the risk of tensions degenerating into violence, EU foreign policy chief Catherine Ashton plans to travel to Kyiv on Tuesday for talks with Yanukovych.

mkg/mz (Reuters, AFP, dpa, AP)