The police dismantled some of the barricades on the perimeter of Independence Square, and many of the protest tents were set on fire. But the 20,000 demonstrators fought back, armed with rocks, bats and fire bombs, and singing the Ukrainian national anthem.
Opposition leader Vitali Klitschko urged the protesters to defend the camp.
"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."
The protesters appeared to sense that Ukraine's political standoff was reaching a critical turning point after the deadliest violence yet was paralyzing the capital.
As the tents went up in flames, defiant protesters shouted "Glory to Ukraine!"
Earlier in the day, protesters attacked police lines and set fires outside parliament, accused President Viktor Yanukovych's government of ignoring their demands once again.
As darkness fell, law enforcement agencies vowed to bring order to the streets and shut down subway stations in 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."
The clashes dimmed hopes for an imminent solution to the political crisis and fueled tensions that began soaring following new steps by Russia and the European Union to gain influence over this former Soviet republic.
Earlier in the day, thousands of angry protesters shouting "Shame!" 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.
Olha Bilyk, spokeswoman for the Kiev city police, told The Associated Press that two policemen were killed, likely by gunshot wounds, in Tuesday's clashes and seven civilians died, including three who were shot.
In addition to the deaths, the Interior Ministry and medics for the protesters said 40 police and about 150 protesters were injured.
Protesters stormed the office of the president's Party of Regions on Tuesday, but police pushed them away. When firefighters arrived to put out a fire, they discovered the body of an office employee, Kiev's emergency services said.
U.S. Ambassador Geoffrey R. Payatt called for dialogue, but 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," Payatt said on Twitter.
The protests began in late November after Yanukovych froze ties with the EU in exchange for a $15 billion bailout from Russia, but the political maneuvering continued and Moscow later suspended its payments. On Monday, however, while opposition leaders were meeting with German Chancellor Angela Merkel, Russia offered a fresh infusion of the billions of dollars that Ukraine needs to keep its ailing economy afloat.
Justice Minister Olena Lukash, a close Yanukovych aide, accused the opposition of violating earlier agreements with the government and blamed protest leaders for the violence.
Tuesday's confrontations came two days after the government and the opposition reached a shaky compromise, with protesters vacating a government building in Kiev they had been occupying since Dec. 1 after the government released of scores of jailed activists.
But tensions rose after Russia's finance minister offered to resume financial aid to Ukraine on Monday, just as Yanukovych was expected to nominate a new prime minister, prompting fears among the opposition that he would tap a Russian-leaning loyalist.
Opposition leader Vitali Klitschko said that Yanukovych agreed to meet with opposition leaders early Wednesday, but admitted that there was little trust in the government left. He called on Yanukovych to agree to the reforms and to call an early election or face a serious escalation of the crisis.
"We are talking minutes, not hours," Klitschko told reporters.
Yanukovych still remains popular in the Russian-speaking eastern and southern regions of Ukraine, where economic and cultural ties with Russia are strong. But western Ukraine is keen to pursue closer ties to the 28-nation EU and move away from Russia's orbit.
Russian President Vladimir Putin promised Yanukovych $15 billion in loans in December, but after purchasing Ukrainian bonds worth $3 billion Russia put the payments on hold. The Russian finance minister said Monday that $2 billion more would be purchased this week.
German Foreign Minister Frank-Walter Steinmeier called on both sides to "refrain from any form of provocation" in order to end the bloodshed.
"An escalation of the violence is the last thing that the country needs," he said in a statement Tuesday.
He said Ukrainian security forces have a "particular responsibility" to de-escalate the situation, adding that the European Union might resort to unspecified sanctions against individuals.
Associated Press writers Yuras Karmanau contributed to this report from Kiev and Laura Mills in Moscow.