"We will not go anywhere from here," Klitschko told the crowd, speaking from a stage in the square as fires burned around him, releasing huge plumes of smoke into the night sky. "This is an island of freedom and we will defend it," he said.
Many heeded his call.
"This looks like a war against one's own people," said Dmytro Shulko, 35, who was heading toward the camp armed with a fire bomb. "But we will defend ourselves."
As police dismantled some of the barricades on the perimeter of the square and tried to push away the protesters, they fought back with rocks, bats and fire bombs. 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. As the tents and also some tires went up in flames, defiant protesters 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.
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.
Tensions had soared after Russia said Monday that it was ready to resume providing the loans that Yanukovych's government needs to keep Ukraine's ailing economy afloat. This raised fears among the opposition that Yanukovych had made a deal with Moscow to stand firm against the protesters and would choose a Russian-leaning loyalist to be his new prime minister.
The protests began in late November after Yanukovych turned away from a long-anticipated deal with the European Union in exchange for a $15 billion bailout from Russia. The political maneuvering continued, however, with both Moscow and the West eager to gain influence over this former Soviet republic.
Until Monday, the government and the opposition had appeared to be making some progress toward resolving the political crisis peacefully. In exchange for the release of scores of jailed activists, protesters on Sunday vacated a government building that they had occupied since Dec. 1.
Russia also may have wanted to see Kiev remain calm through the Winter Olympics in Sochi, so as not to distract from President Vladimir Putin's games. But after the outburst of violence against riot police, Yanukovych's government may have felt it had no choice but to try to restore order.
While Kiev and western Ukraine have risen up against Yanukovych, he remains popular in the Russian-speaking eastern and southern regions, where economic and cultural ties with Russia are strong.
As darkness fell, law enforcement agencies vowed to bring order to the streets and they shut down subway stations in the center of the capital. In Independence Square, Orthodox priests prayed for peace.
"We see that this regime again has begun shooting people; they want to sink Ukraine in blood. We will not give in to a single provocation," opposition leader Arseniy Yatsenyuk told the protesters. "We will not take one step back from this square. We have nowhere to retreat to. Ukraine is behind us, Ukraine's future is behind us."