Against the backdrop of a soaring monument to Ukraine's independence, protesters fed the burning flames with tires, creating walls of fire to prevent police from advancing.
A large building the protesters had used as a headquarters caught fire and many struggled to get out.
Many of the protesters were bleeding.
Speaking over loudspeakers, police urged women and children to leave the square because an ‘‘anti-terrorist'' operation was underway.
The protesters appeared to sense that Ukraine's political standoff was reaching a critical turning point. Waving Ukrainian and opposition party flags, they shouted ‘‘Glory to Ukraine!'' and sang the Ukrainian national anthem.
Shortly before midnight, Klitschko headed to President Viktor Yanukovych's office to try to resolve the crisis, his spokeswoman said. An hour later, he was still waiting to be received.
Earlier in the day, protesters attacked police lines and set fires outside parliament, accusing Yanukovych of once again ignoring their demands and dragging his feet on a constitutional reform to limit presidential powers.
