The police dismantled some of the barricades on the perimeter of Independence Square in Kiev, and many of the demonstrators’ tents were set on fire.
However, the 20,000 protesters fought back, armed with rocks, bats and fire bombs, and singing the Ukrainian national anthem.
Opposition leader Vitali Klitschko urged protesters to defend the camp.
“We will not go anywhere from here,” he 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 the call from the former boxing world champion.
“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 paralysing Kiev.
As tents went up in flames, protesters shouted: “Glory to Ukraine!”
Earlier in the day, protesters attacked police lines and set fires outside parliament, accusing 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 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 fuelled tensions that began soaring following new steps by Russia and the EU 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, said six policemen were killed, likely by gunshot wounds, in yesterday’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, 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.
US 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,” Mr Payatt said.
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.