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 21 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 met with President Viktor Yanukovych for talks, but the meeting failed to produce a breakthrough. Klitschko said the president demanded protesters "stop the standoff" and unconditionally clear the square.
He told the Ukrainska Pravda website that he brings "nothing good from the talks."
Mr. Yanukovych's advisers told reporters the president will address the nation after meeting with the opposition on Wednesday. The exact time for the address has not been announced.
U.S. Vice President Joe Biden phoned Mr. Yanukovych late Tuesday 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 the national anthem.
Police confirmed the unrest has spread outside Kyiv to a number of regions in western Ukraine that support closer ties to the European Union. In Lyiv, anti-government demonstrators seized the regional administration building and police headquarters.
In Moscow, authorities blamed the violence on Western governments, accusing them of encouraging "radical forces" among the protesters.
Russia said Monday 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.
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.