– Ominous new action by Ukraine’s security forces Monday, including a raid on an opposition party and threats of treason charges, appeared to scuttle an opening for talks between the government and demonstrators, as Western leaders grasped to defuse the country’s intensifying political crisis, witnesses and opposition figures said.

In a sign of renewed alarm, U.S. Vice President Joe Biden and President Jose Manuel Barroso of the European Commission each made calls to Ukraine’s besieged president, Viktor Yanukovych, to warn him away from unleashing violence on a mass demonstration movement in its third week. And senior envoys including the European foreign affairs chief, Catherine Ashton, and U.S. Assistant Secretary of State Victoria Nuland were being sent to try to defuse the crisis.

After seeming to lose control of Kiev on Sunday night following a huge rally of hundreds of thousands of people in Independence Square, police forces redeployed on Monday and began efforts to push protesters out of streets near main government buildings. Battalions of police officers moved in and took up positions just outside the square’s perimeter.

Then, Monday evening, the Ukrainian security service raided the headquarters of the opposition Fatherland Party and seized computer servers.

The party’s parliamentary leader, Arseniy Yatsenyuk, is one of the main organizers of the protest movement, which has ballooned in recent days to dominate the streets of Kiev and put pressure on Yanukovych after he refused to sign a political and trade pact with the European Union. But the party is best known as the opposition coalition formed by the jailed former prime minister, Yulia Tymoshenko, whose release has long been demanded by Western leaders.

“They came without any notice, without any explanations, fully armed,” said Natalia Lysova, a spokeswoman for Fatherland who often accompanies Tymoshenko’s daughter, Evgenia, at public appearances. “They broke the door, took all the servers and left.” Lysova said the security officers did not arrest anyone.

Just hours before the raid, Yanukovych had signaled that he would accept a proposal by three predecessors to hold “a national round table for finding a compromise” and that initial discussions would begin Tuesday. But any sense that his willingness to negotiate might defuse the crisis was quickly erased.

“We saw on the Internet today some statement about the round table,” Yatsenyuk said. “We would like to start by saying that it is very difficult to fit a round table into a square cell.”