Story highlights Masked men raid opposition party's headquarters, its leader says

Opposition leader Vitaly Klitschko urges police to stay calm

U.S. Vice President calls President Viktor Yanukovych, urging talks with opposition

Ukraine president supports proposal by a predecessor for "nationwide panel discussion"

Protesters in Ukraine remained defiant and continued their mass demonstration against President Viktor Yanukovych over the country's U-turn away from the European Union Monday, the same day masked men raided the headquarters of the opposition party Monday, according to opposition party leader Arseniy Yatsenyuk.

"Members of the special ops destroyed the entire server room," he said. "Equipment was destroyed, dragged out, across the entire premise of the Batkivshchyna Political Party headquarters." Police denied any involvement in the raid.

The raid happened after Yanukovych 's announcement on his website that he would back a call for talks involving the opposition to work out a compromise.

With pressure growing, thousands remained on the streets of Kiev, some facing off with lines of Interior Ministry troops Monday evening near the presidential administration building. Minor scuffles broke out, but demonstrators were holding their ground as security forces began removing barricades.

Opposition leader and former heavyweight champion Vitaly Klitschko approached riot police and urged them to stay calm and "not break the law" should they be ordered to remove protesters.

"None of us has either guns nor other objects; this is a peaceful protest," he said.

U.S. Vice President Joe Biden entered the fray Monday with a phone call to Yanukovych in which he "expressed his deep concern about the situation in Ukraine and the growing potential for violence," according to a statement.

Biden said he urged Yanukovych to hold talks with the opposition.

Burning tires to stay warm

Faced with freezing temperatures as the bitter cold sets in, the protesters burned tires and sipped hot soup and tea to stay warm.

Some played soccer or strummed guitars as they camp out in tents. The crowds often swell in the evenings as people leave work and join the rallies.

As more police gathered on the streets, the demonstrators received a message of support from actor George Clooney.

"We here in the United States have great affinity for those seeking democracy," Clooney said in a video posted online.

"We learned through trial and error that true democracy cannot exist without a free and fair and honest election."

The actor, who has lent his voice to campaigns for various issues in the past, added: "Let me just say this to all of you in the square in Kiev or all around Ukraine: When you look to the West, know that we are looking back at you with great admiration."

Populist movement

The protests began when Kiev refused a deal with the European Union, opting instead for closer ties with neighbor Russia.

It has grown into a populist movement, the biggest the Eastern European country has seen since the so-called Orange Revolution toppled the government nine years ago.

"The government and opposition should hold talks to solve this. It has gone too far, it might result in conflict," Vysotsky said. "We do not want a conflict."

European Commission President Jose Manuel Barroso said EU foreign policy chief Catherine Ashton would travel to Kiev on Tuesday to try to "bring some solutions to the very tense situation that Ukraine is living today."

Speaking at a conference in Milan, Italy, Barroso said he had spoken with Yanukovych by phone Sunday.

"I asked him to show restraint in the face of these recent developments, to not use force against the people that are demonstrating peacefully, to respect fully the freedoms that are so important for all of us in Europe," he said.

U.N. Secretary-General Ban Ki-moon called Yanukovych on Sunday and told the Ukrainian President that he had "grave concern" about the situation, urging authorities not to resort to violence. Yanukovych told the U.N. chief that "consultations would be initiated to defuse the situation," the United Nations said.

East vs. West

The protesters say an EU agreement would have opened borders to trade and set the stage for modernization and inclusion. They accuse Yanukovych , who met Russian President Vladimir Putin on Friday, of preparing to take the country into a Moscow-led customs union.

The tensions tugging at the country are felt across the nation -- Ukraine is split between pro-European regions in the west of the country and a more Russia-oriented east.

One of the main reasons for Yanukovych 's decision to backpedal on the EU talks is Russia's threat of trade sanctions and gas bill hikes. Yanukovych was also under pressure by the EU to free Tymoshenko, his jailed chief political opponent.

The Orange Revolution that swept him from office in 2004, when he was Prime Minister, also swept Tymoshenko to power.

Tymoshenko was sentenced to seven years in prison in 2011 after being convicted of abuse of authority over a natural gas deal negotiated with Russia in 2009. The United States and Europe see the punishment as politically motivated.

Many of the protesters have carried her picture in Independence Square during the rallies.

"This is the end of Soviet occupation," the party's Twitter account said. "End of (the) regime of shame and humiliation."