Updated

Heavily armed riot troops stormed the headquarters of a top Ukrainian opposition party in Kiev and stole computer servers, the party said Monday, as anti-government protests crippled the capital for yet another day.

A spokeswoman for the party of jailed opposition leader Yulia Tymoshenko blames police for the raid, but police have denied any involvement according to Reuters.

Ostap Semerak told The Associated Press that troops broke into the Fatherland Party's offices on Monday evening. He said some troops were walking along its corridors while others were climbing in through the windows. He called the situation "insane."

Tensions also rose as a double cordon of helmeted, shield-holding police deployed in the street near Kiev's city administration building, which demonstrators had occupied and turned into a makeshift command post and dormitory.

Early Tuesday, electricity to the building was cut off and occupiers began leaving, some carrying out blankets and other goods, expecting that police were preparing to storm the site. But a small crowd remained on the steps and in the street. About three hours later, the lights came back on and some of the protesters returned to occupy the building.

Also, many cellular phones in the area received a text message early Tuesday addressed to protesters reading in part "You are surrounded, there are no chances." The number from which the message was sent rang to an unidentified voicemail.

President Viktor Yanukovych has faced three weeks of protests after his decision to freeze ties with the European Union and align himself with Russia.

The protests were galvanized after police violently dispersed some of the demonstrators. Sunday's demonstration by hundreds of thousands was the largest since the 2004 pro-democracy Orange Revolution protests that annulled Yanukovych's presidential victory due to voting fraud.

In a surprise move, Yanukovych announced that he would sit down with three former Ukrainian presidents on Tuesday to discuss a way out of the crisis. The European Union's foreign policy chief, Catherine Ashton, was headed to Ukraine to help defuse the tensions.

The political standoff has been aggravated by Ukraine's deteriorating finances. The economy has been in recession for more than a year, and the government is in desperate need of foreign funding to avoid a default. As talks stalled with the International Monetary Fund, Yanukovych has sought a bailout loan from Russia.

At the square, black-robed Orthodox priests sang solemn prayers Monday calling for peace amid heavy snowfall. Some talked to the police.

Opposition leader Arseniy Yatsenyuk called for calm, telling several thousand protesters on Independence Square that police were ordered not to storm the building but to blockade the protest camp to deplete it of food and other amenities.

"I am turning to all Ukrainians: You must all go to the heart of the Maidan," he said.

Some activists approached police lines, urging officers to come over to their side and even offering them food.

As tensions mounted, Yanukovych announced on his website that he would meet with Ukraine's three former presidents to discuss the situation. But at the same time, prosecutors called in several opposition leaders for questioning.

Ukraine's political standoff has been aggravated by its rapidly deteriorating finances. The economy has been in recession for more than a year, and the government is in desperate need of foreign funding to avoid a default. As talks stalled with the International Monetary Fund, Yanukovych has sought a bailout loan from Russia.

This former Soviet republic of 46 million people is sharply divided over the prospects of drawing closer to its powerful neighbor, Russia. Yanukovych's stronghold in eastern Ukraine, the country's industrial heartland, is dominated by Russian speakers who want closer ties to Russia, in contrast to Kiev's students and residents in the west who see better EU ties as the way forward.

Opinion polls, however, show that the EU is more popular among Ukrainians than Russia.

Wearing helmets and holding shields, Ukrainian police surrounded three tent encampments outside the government and presidential offices in central Kiev on Monday night. Riot police also began removing barricades on the approach to the government building. Most protesters remained standing.

World boxing champion and opposition leader Vitali Klitschko warned the authorities against any further escalation in tensions.

"We are calling upon law enforcement to restrain from using force against peaceful demonstrators," he said as he tried to stop police from removing the tents.

A large protest test camp remained in place on Independence Square, the downtown plaza that is the epicenter of the protests.

The square is a few hundred yards (meters) from the protester-occupied city administration building, which a court has ordered demonstrators to vacate by Monday.

The appearance of riot police nearby raised anxieties inside, and some of the hundreds of people inside left.  But hundreds remained inside, armed with wood planks, metal rods and bottles of sunflower oil, hoping to make riot police slip if they advanced.

"We won't let anybody into the building," said Vasyl Khlopotaruk, one of the activists. "But we hope there isn't bloodshed."

Some activists approached police lines, urging officers to come over to their side and even offering them food.

As tensions mounted, Yanukovych announced on his website that he would meet with Ukraine's three former presidents to discuss the situation: Leonid Kravchuk, Leonid Kuchma and Viktor Yushchenko. But at the same time, prosecutors called in several opposition leaders for questioning.

EU Commission President Jose Manuel Barroso dispatched EU foreign policy chief Ashton to Kiev on Tuesday, saying she will try to help defuse "the very tense solution that Ukraine is living today." Barroso praised the demonstrators, saying they are "writing the new narrative for Europe."

U.S. Ambassador to Ukraine Geoffrey Pyatt cautioned the government against using force.

"Peaceful demonstrations must be allowed to continue," he wrote on Twitter. "Dialogue and non-violence key, world watching. Opportunity must not be lost."

U.S. Vice President Joe Biden spoke with Yanukovych by phone Monday and urged him to defuse tensions and begin talks with opposition leaders, the White House said.

The protests that erupted on Nov. 21 have had an anti-Russian bent because Moscow worked hard to derail the Ukraine-EU deal, issuing threats of trade consequences.

On Sunday, hundreds of thousands of protesters calling for Yanukovych's ouster poured into Kiev, toppling a statue of former Soviet leader Vladimir Lenin and blockading government buildings.

Protesters on Monday vandalized another Lenin statue in the southern town of Kotovsk.

"Only the legs are left standing," town spokeswoman Yelena Khaustova told the AP.

The Associated Press and Reuters contributed to this report