Last Updated Feb 18, 2014 11:40 PM EST
KIEV, Ukraine - Amid cries of "Glory to Ukraine!" and with flaming tires lighting up the night sky, thousands of riot police armed with stun grenades and water cannons attacked the sprawling protest camp in the center of Kiev on Tuesday, following a day of street battles that left 20 people dead and hundreds injured.
The violence was the deadliest in nearly three months of anti-government protests that have paralyzed Ukraine's capital in a struggle over the nation's identity, and the worst in the country's post-Soviet history.
The building the opposition has been using as its headquarters in Kiev's Independence Square was on fire late Tuesday, Reuters TV reported. Live footage from the square showed several floors of the nine-story Trade Union building were on fire with flames and smoke streaming from the windows.
Vice President Joe Biden telephoned Ukrainian President Viktor Yanukovych and urged him to "pull back government forces and to exercise maximum restraint," the White House said in a statement.
"We will not go anywhere from here," Klitschko told the crowd, speaking from a stage in the square as fires burned around him, releasing huge plumes of smoke into the night sky.
"This is an island of freedom and we will defend it," he said.
Many heeded his call.
"This looks like a war against one's own people," said Dmytro Shulko, 35, who was heading toward the camp armed with a fire bomb. "But we will defend ourselves."
Shortly before midnight, Klitschko headed to Yanukovych's office to try to resolve the crisis. He returned to the square early Wednesday without reaching any agreement on ending the violence.
Klitschko told reporters that he had asked the president to stop the police action to clear the square and prevent further deaths, but Yanukovych's only proposal was that the demonstrators have to go home and stop the protests.
"I am very unhappy because there was no discussion," Klitschko said. "They don't want to listen."
As police dismantled some of the barricades on the perimeter of the square and tried to push away the protesters, they fought back with rocks, bats and fire bombs. Many of the protesters were bleeding.
Speaking over loudspeakers, police urged women and children to leave the square because an "anti-terrorist" operation was under way.
The action late Tuesday was taking place in what has been the epicenter of nearly three months of anti-government protests on the edge of Kiev's Independence Square on Tuesday.
Western powers warned President Yanukovich against trying to smash the 12 week-old pro-European demonstrations. Earlier, Klitschko, fearing an assault, urged women and children to leave the central square - also known as Maidan - “to avoid further victims."
Earlier, the State Security Service, or SBU, issued a joint statement with the Interior Ministry setting a 6 p.m. deadline for protesters to end street disorder or face “tough measures."
Forces loyal to the Russian-backed president broke through front-line barricades near the Dynamo Kiev soccer stadium and advanced to the edge of the occupied Independence Square. They were met after nightfall with a hail of petrol bombs and fireworks, responding with bursts of water cannon.
Fires lit by protesters raged on the fringes of the square to prevent police advancing as opposition speakers harangued the crowd, interspersed with patriotic music.
The riot police moved in hours after Moscow gave Ukraine $2 billion in aid which it had been holding back to demand decisive action to crush the protests.
Nationwide protests against Yanukovich erupted in November after he bowed to Russian pressure and pulled out of a planned far-reaching trade agreement with the European Union, deciding instead to accept a Kremlin bailout for the former Soviet republic's heavily indebted economy.
EU Enlargement Commissioner Stefan Fuele said he had spoken to Ukraine's acting prime minister, who had given assurances that the authorities would try to avoid using live firearms.
“For the sake of the Ukrainians and for the sake of the future of that country, I will pray that he is right,” Fuele told a public event in Brussels, Belgium.
In what has become a geo-political tussle redolent of the Cold War, the United States and its Western allies are urging Yanukovich to turn back to Europe and the prospect of an IMF-supported economic recovery, while Russia accuses them of meddling.
As protesters and police battled on the streets of Kiev, Russia called the escalation a “direct result of connivance by Western politicians and European structures that have shut their eyes ... to the aggressive actions of radical forces."
EU foreign policy chief Catherine Ashton, who has tried to broker a power-sharing transition, urged Ukraine's leadership “to address the root causes of the crisis".
In a statement, Secretary of State John Kerry said the U.S. condemns "the use of force or violence by any party," adding: "We call on the government and protesters to take immediate steps to de-escalate the situation and to resolve political differences through high-level dialogue."
Monday's $2 billion cash injection, a resumption of the $15 billion aid package, was seen as a signal that Russia believes Yanukovich has a plan to end the protests and has dropped any idea of bringing opposition leaders into government.
In another apparent gesture
toward Moscow, a Ukrainian government source said state gas company Naftogaz
has paid back $1.3 billion of its 2013 debt to Russian gas monopoly Gazprom,
although it still owes $1.5 billion.