Fires burned and stun grenades ripped through the center of the Ukrainian capital early Wednesday as riot police charged the main opposition protest camp after clashes killed at least 18 people, including seven police officers.
Police and opposition representatives said many of the dead were killed by gunshots and hundreds more injured, with dozens in serious condition.
Ukrainian opposition leader Vitali Klitschko urged the pro-Western demonstrators occupying Kyiv's Independence Square - also known as Maidan - to defend their positions. He warned women and children to leave the area.
Later, Klitschko was to begin emergency talks with President Viktor Yanukovych in an attempt to resolve the deadly crisis, Ukraine's bloodiest since it gained independence from the Soviet Union in 1991.
U.S. Vice President Joe Biden phoned Mr. Yanukovych to express "grave concern" about the violence, calling on the Ukrainian president to pull back security forces and to exercise maximum restraint.
Biden told Mr. Yanukovych his government bears "special responsibility" to resolve the crisis.
Anti-government protests have been building for weeks, with activists calling for Mr. Yanukovych's ouster after he backed away from a trade deal with the European Union in favor of closer ties with Russia.
Protesters, some armed with clubs and wearing helmets and body armor, attempted to stand their ground in central Kyiv, hurling firebombs and stones at police as plumes of smoke billowed from burning tents and piles of tires and wood.
Security forces have been steadily gaining ground in the square, where thousands of protesters still remained, hearing speeches from their leaders and singing Ukraine's national anthem.
In the largely pro-Western city of Lyiv, anti-government demonstrators seized the regional administration building and police headquarters.
White House spokesman Jay Carney said Tuesday the United States is "appalled" by the violence in Ukraine. He urged Mr. Yanukovych to "de-escalate" the situation.
Earlier, Ukraine's Interior Ministry and state security agency said they would be "forced to introduce order through all legal means" if unrest did not end by 6 p.m., local time .
European Union foreign policy chief Catherine Ashton said Tuesday she was "deeply worried" by the "grave new escalation" in Kyiv and condemned "all use of violence, including against public or party buildings."
Ashton urged Ukraine's political leaders "to address the root causes of the crisis."
In Moscow, authorities blamed the violence on Western governments, accusing them of encouraging "radical forces" among the protesters.
Russia Monday said it would release an additional $2 billion to Ukraine to support its moribund economy, a move seen as strengthening President Yanukovych's bid to remain in power. The money is part of a $15 billion loan promised by Russia.
Weeks of protests in Kyiv and other major Ukrainian cities have spawned widespread calls for more democracy in the former Soviet republic. German Chancellor Angela Merkel, a leading figure in the 28-member EU trade bloc, met Monday with Klitschko and another Ukrainian opposition leader, Arseni Yatsenyuk.
European Union and U.S. officials have repeatedly said they are working with the International Monetary Fund on details of an aid package that analysts have described as rivaling or exceeding the Russian bailout deal. No concrete offers have been made public.