Ukrainian President Viktor Yanukovych has promised to talk with the opposition after a day of bloody violence.
More than 100,000 anti-government demonstrators marched in Kyiv Sunday against new laws restricting protests.
Radical activists wearing hard hats and gas masks threw rocks and gasoline bombs at the outnumbered riot police, who responded with stun grenades and water cannons in sub-freezing weather. Dozens of people were hurt.
Opposition leader Vitaly Klitschko pleaded with the crowd to avoid violence, but was sprayed in the face with a fire extinguisher and shouted down.
He later met with Yanukovych, who appointed a working group to address opposition complaints.
Anti-government protesters have filled the streets of Kyiv and other Ukrainian cities since November, when Yanukovych backed out of a plan to sign a trade deal with the European Union in favor of stronger ties with Russia.
A White House spokeswoman is calling for calm. She calls the tension a direct consequence of the Ukrainian government's failure to acknowledge the people's legitimate grievances.