Ostap Semerak, a member of the Fatherland Parry, told The Associated Press that troops broke into the party's offices on Monday evening, some climbing in through its windows.
"They are storming us. The images are insane," he said by telephone.
The troops left after confiscating some computer equipment, he said. An Associated Press reporter later saw broken glass and smashed computers in the offices.
Party member Marina Soroka also said the troops surrounded and blockaded several opposition-minded Ukrainian media outlets, making their and other media websites inaccessible.
The party is headed by imprisoned former Prime Minister Yulia Tymoshenko, a longstanding foe of President Viktor Yanukovych, and is the largest opposition grouping in the parliament. Critics say Tymoshenko's conviction on abuse of office charges was a case of political revenge.
In a surprise move, Yanukovych announced he would sit down with three former Ukrainian presidents on Tuesday to discuss a way out of the crisis that has paralyzed the country. The European Union's foreign policy chief, Catherine Ashton, also was heading to Ukraine to help defuse the tensions.
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 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, 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.
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Associated Press writer Maria Danilova contributed to this report.
Follow Danilova and Heintz on Twitter at https://twitter.com/mashadanilova and https://twitter.com/jeheintz
