KIEV, Ukraine - Heavily armed riot troops broke into the offices of a top Ukrainian opposition party in Kiev and seized its servers Monday, the party said, as antigovernment protests crippled the capital for yet another day.

Elsewhere, police dismantled or blocked off several small protest tent camps that near key government buildings in the city.

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.

The moves came a day after hundreds of thousands of demonstrators crammed into Kiev, the biggest in three weeks of protests that started when Ukraine's president backed away from signing a long-awaited pact to deepen ties with the 28-nation European Union.

Protesters are angered not only by the thwarting of their desire to become closer to the West and spin out of Russia's orbit, but also by police violence against the demonstrators. Club-swinging police have twice broken up protest rallies.

Ostap Semerak, a member of the Fatherland Party, said 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.

In a surprise move, President Viktor 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 tensions.