KIEV, Ukraine -- With the boom of exploding stun grenades and fireworks drowning out his words at times, Ukraine opposition leader and former heavyweight boxing champion Vitali Klitschko urged 20,000 protesters Wednesday to defend the camp that has been the heart of protests after a day of street battles left at least 25 people dead and hundreds injured.
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 on Independence Square in the center of Kiev.
"We will not go anywhere from here," Klitschko 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."
Meanwhile, in Sochi, Sergei Bubka, the pole vault great who heads the Ukrainian Olympic Committee, urged both sides to lay down their weapons and halt the violence that is bringing the country to "the brink of catastrophe."
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.
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.
Bubka, who is in Sochi as an executive board member of the International Olympic Committee and president of Ukraine's national Olympic body, appealed to all parties involved in the conflict to resume negotiations.
"I'm shocked by what is happening in my native country -- especially because the violence is taking place during the Olympic Games, the world's most peaceful and democratic event," Bubka said in a statement.
"I am once again urging all parties to stop the violence which puts our country on the brink of catastrophe. There is no 'their' Ukraine, or 'your' Ukraine. It is our Ukraine. Let's do everything possible to get back to negotiations and make a compromise. ... There is still a chance for a peaceful solution."
After denying a request Wednesday by the Ukraine Olympic Committee to wear black armbands as an "expression of sorrow and sympathy" to honor those who have died during the protests, IOC president Thomas Bach expressed his condolences to the victims of the violence.
"Our thoughts and sympathy are with the Ukrainian team at what must be a very difficult time," Bach said. "The way they have continued to represent their nation with great dignity is a credit to them and their country. Their presence here is a symbol that sport can build bridges and help to bring people from different backgrounds together in peace."
The violence began Tuesday when protesters attacked police lines and set fires outside parliament, accusing Yanukovych of 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.
"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," 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."
Tuesday's clashes were the first to lead to deaths since Jan. 22, when two protesters were hit with live ammunition and a third died after a fall. As angry protesters outside parliament hurled stones at police and set trucks blocking their way on fire, riot police retaliated with stun grenades and fired what appeared to be small metal balls, as smoke from burning tires and vehicles billowed over Kiev.
In Washington, Vice President Joe Biden expressed his "grave concern" in telephone call to Yanukovych, urging him to pull back government forces and exercise maximum restraint. The White House said Biden also called on Ukraine's government to address the protesters' "legitimate grievances" and put forward proposals for political reform. Earlier, U.S. Secretary of State John Kerry urged both sides to end the violence, halt their ultimatums and hold high-level talks. U.S. Ambassador Geoffrey R. Pyatt also threatened both sides with sanctions.
"We believe Ukraine's crisis can still be solved via dialogue, but those on both sides who fuel violence will open themselves to sanctions," Pyatt said on Twitter.
Information from The Associated Press was used in this report.