KIEV, Ukraine — 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 P. Yatsenyuk, is one of the main organizers of the protest movement, which has ballooned in recent days to dominate the streets of Kiev and pressure 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 V. 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.
A day earlier, the security service, known as the SBU, issued a curt statement saying that it had opened an investigation into possible treason charges against unnamed politicians. At a news conference with other protest leaders Monday, Yatsenyuk said he had been summoned for questioning on Tuesday.
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 ñell."
He added: "We understood that this is the way Yanukovych invites us to the round table — a few thousands of Interior forces have arrived already, and I received a summon for interrogation from the General Prosecution Office."
The raid and police remobilization brought a fresh round of warnings from Western leaders, who reacted in alarm after the security forces violently cracked down on protesters on Nov. 30.
White House officials said Biden pressed Yanukovych to immediately de-escalate the crisis and open talks with opposition leaders. And he warned that "violence has no place in a democratic society and is incompatible with our strategic relationship," according to a White House summary of the leaders' phone conversation.
Ashton, the European Union's envoy, was to arrive Tuesday. She had been deeply involved in efforts to draw Ukraine into closer ties with the European Union through the trade deal that Yanukovych abruptly backed away from late last month.
That decision to abandon a wide-ranging trade and political agreement with the European Union left officials in Brussels deeply frustrated and upended years of efforts to draw Kiev into Europe's orbit and away from Russia's.
Warning that "anti-European" voices risk reviving old "demons of Europe, like extreme nationalism, like xenophobia," Barroso, the European Commission president, praised Ukrainian protesters for showing that Europe has positive values that make it attractive as a model.
"If sometimes in Europe some of us have doubts about how important these values are, just look at Ukraine," he said "Those young people in the streets of Ukraine, with freezing temperatures, are writing the new narrative for Europe."
The security forces' maneuvering Monday came amid a driving snowstorm that added a frigid layer of chaos to this churning capital.
The growing protest movement here has largely been pushed by a grass-roots coalition of civic organizations and student groups, and many of the rank-and-file demonstrators are deeply skeptical that organized politicians will be able to deliver the changes that they seek.
Despite the action against Yatsenyuk's Fatherland Party, authorities seemed to be holding back from similar investigations of the other two parliamentary leaders at the forefront of the protests, the champion boxer Vitali Klitschko, of the Udar Party; and Oleg Tyganibok, of the nationalist Svoboda Party. Tyagnibok's supporters in particular are among the most fearsome involved in the demonstrations and have led some of the more provocative efforts to occupy buildings and block government offices.
At their news conference, the opposition leaders reiterated their demands: the release of demonstrators who have been arrested, the punishment of government officials responsible for the violent crackdown on demonstrators on Nov. 30, and the dismissal of Prime Minister Mykola Azarov and the rest of the government.
Those demands put them at odds with not only Yanukovych but also with Ukraine's first president, Leonid Kravchuk, who was given credit by Yanukovych's office for pushing the idea of round-table discussions.
Yanukovych's three predecessors — Kravchuk, Leonid D. Kuchma and Viktor A. Yushchenko — issued a statement last week calling for "round-table" talks, using the same phrase that described negotiations that helped resolve Ukraine's 2004 Orange Revolution.
In the statement, the three sharply criticized the government for what they called the "hasty and spontaneous decision" to abandon sweeping political and free trade agreements with the European Union that Yanukovych had been promising that he would sign for more than a year. "A feeling of being cheated gives the rallies special emotional color and deepens distrust of the authorities," they said.
They also criticized the crackdown on peaceful protesters on Nov. 30, calling the bloodshed unprecedented in Ukraine's near quarter-century as an independent nation.
"The brutality with which the special police acted should be condemned not only publicly but also punished according to the Ukrainian legislation as totally unacceptable in a democracy," the former presidents wrote.
The announcement of possible negotiations was the first glimmer of engagement by Yanukovych after more than two weeks in which he largely waved off the growing national uproar.
Even as Kiev convulsed in anger, he traveled to China on a state visit, then stopped in Russia to meet with President Vladimir Putin, further enraging the crowds already mad about the Kremlin's role in pressuring Yanukovych to abandon the far-reaching political and free trade agreements with the European Union.
In other developments Monday, Ukrainian authorities shut three main subway stations, including the two located directly under the main protest site. In a statement posted on the Interior Ministry website, officials said they had acted in response to bomb threats, and the stops were later reopened.
But as police buses moved into lines just outside the perimeter of Independence Square, and police officers took up new positions, demonstrators were bracing for an enforcement action. They scrambled to reinforce barricades, moving public benches, wood planks and anything else available, to add to the fortifications that have closed off the area for more than a week.
Rumors that police would emerge in force from the subway led to new barricades at the station entrances. Some demonstrators appeared to evacuate Kiev City Hall, which they had occupied, in the expectation that it would be an early target of any police action. A crowd gathered outside, including many television news crews, indicating that whatever might happen was likely to be highly publicized.