Thousands of police armed with stun grenades and water cannons have clashed with anti-government protesters in the centre of Ukraine's capital, Kiev, and police say at least 13 people have been killed.
Moving into Independence Square, which has been the centre of nearly three months of protests, police on Tuesday dismantled some of the barricades and many of the protesters' 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.
Police said six officers were killed in the clashes, and seven civilians were reported killed, including three who were shot.
Opposition leader Vitali Klitschko had urged the protesters to defend the camp.
"We will not go anywhere from here," Mr 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.
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.
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 European Union to gain influence over this former Soviet republic.
In addition to the deaths, the Interior Ministry and medics for the protesters said 40 police and about 150 protesters were injured.
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 on Twitter.
The protests began in late November after Mr Yanukovych froze ties with the EU in exchange for a 15 billion US dollars (£9bn) bailout from Russia, but the political manoeuvering 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.
Today'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 December 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, just as Mr Yanukovych was expected to nominate a new prime minister, prompting fears among the opposition that he would choose a Russian-leaning loyalist.
Mr Klitschko said that Mr Yanukovych agreed to meet opposition leaders early on Wednesday, but admitted that there was little trust in the government left. He called on Mr Yanukovych to agree to the reforms and to call an early election or face a serious escalation of the crisis.
Mr 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.