Editor's Note: Watch a live stream of the protests in Kiev, Ukraine.
KIEV, Ukraine (AP) -- Amid cries of "Glory to Ukraine!" and with flaming tires lighting up the night sky, thousands of riot police armed with stun grenades and water cannons attacked the sprawling protest camp in the center of Kiev, following a day of street battles that left at least 18 people dead and hundreds injured.
The violence on Tuesday was the deadliest in nearly three months of anti-government protests that have paralyzed Ukraine's capital in a struggle over the identity of a nation divided in loyalties between Russia and the West, and the worst in the country's post-Soviet history
With the boom of exploding stun grenades and fireworks nearly drowning out his words at times, opposition leader Vitali Klitschko urged the 20,000 protesters to defend the camp on Independence Square that has been the heart of the protests.
"We will not go anywhere from here," Klitschko, a former heavyweight boxing champion, told the crowd, speaking from a stage in the square as tents and tires burned around him, releasing huge plumes of smoke. "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. 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. He returned to the square early Wednesday without reaching any agreement on ending the violence. Klitschko told reporters that he had asked the president to stop the police action to clear the square and prevent further deaths, but Yanukovych's only proposal was that the demonstrators have to go home and stop the protests.
"I am very unhappy because there was no discussion," Klitschko said. "They don't want to listen."
Still, Klitschko urged the protesters and police to stop the escalation of violence. He said opposition leader Arseniy Yatsenyuk was trying to arrange for more negotiations with Yanukovych later Wednesday.
The violence began Tuesday when protesters attacked police lines and set fires outside parliament, accusing Yanukovych of once again ignoring their demands. Parliament, dominated by his supporters, was stalling on taking up 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.