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 President Viktor Yanukovych addressed the nation Wednesday, saying activists who urged others protesters to bring weapons to the anti-government demonstration in central Kyiv have disregarded the principles of democracy and must face legal repercussions.
Ukrainian opposition leader Vitali Klitschko met with President 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 urged the pro-Western demonstrators occupying Kyiv's Independence Square - also known as Maidan - to defend their positions.
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.