Riot police and antigovernment protesters continued to clash overnight in Kyiv.
At least 18 people, including seven police, were killed and hundreds wounded on February 18 in the Ukrainian capital in the worst day of violence since protests against President Viktor Yanukovych erupted last November.
Overnight, television footage showed opposition activists huddled on Independence Square, or Maidan, encircled by fires and smoke from burning tents as fighting with riot police moved closer.
Several floors of a trade union building, used as an anti-government headquarters, burned in Kyiv.
Opposition leaders Vitali Klitschko and Arseniy Yatsenyuk met with Yanukovych early February 19, but Klitschko said afterwards that the talks had failed to reach a compromise.
Klitschko said Yanukovych refused to pull back riot police massed against thousands of protesters in Independence Square.
"I am very unhappy because it was no discussion and the President doesn't want to listen to the opposition. They don't want to listen, it's just one way and the opposition and all protesters have to stop protests, have to stop demonstration he said. But right now it's very important to make a break and not fight any more," Klitschko told reporters.
Klitschko has urged protesters to remain on Independence Square, calling it an "island of freedom."
Yanukovych urged leaders of the opposition to distance themselves from "radicals," warning if they don't, Yanukovych will "talk differently" with them.
In a statement early on February 19, Yanukovych said some members of the opposition had crossed a line when they allegedly urged supporters to bring weapons to the demonstration in the central square of Kyiv.
The president called those people "criminals" and said they would face justice in court. Still, Yanukovych said it was "not too late to end the conflict."
Earlier, U.S. Vice President Joe Biden called on Yanukovych to pull back riot police and exercise maximum restraint.
That message was echoed earlier by White House Press Secretary Jay Carney.
"We are appalled by the violence that was already taking place in downtown Kyiv and reports of armed riot police massing on the edge of Maidan. We continue to condemn violence and excessive use of force by either side. Force will not resolve the crisis. To restore peace and stability, we urge President Yanukovych to de-escalate immediately the situation and end the confrontation at Maidan," Carney said.
EU foreign policy chief Catherine Ashton urged Yanukovych to "address the root causes of the crisis."
Meanwhile, reports say protesters have stormed government buildings and police centers in several cities in western Ukraine including Lviv, Ivano-Frankivsk and Ternopol.
And ITAR-TASS reports dozens of protesters have torched piles of tires to block a car border crossing between Poland and Ukraine in the Lviv region.
Ukrainian Prosecutor-General Viktor Pshonka said in a televised statement from his office that the opposition march "grew into an armed confrontation" and said "leaders of the opposition should take all responsibility for everything that is happening today on the streets of Kyiv."
WATCH: RFE/RL Ukrainian Service video of brutal clashes between police and protesters in downtown Kyiv on February 18.
"Organizers of mass riots will be held accountable," he added. "The Prosecutor-General's Office will demand the heaviest punishment both for those who revved people up to take part in today's action and for those who organized and directed them."
EU and American officials have expressed shock at the dramatic turn of events and appealed for a nonviolent end to the unrest, which follows three months of intense street protests and periodic crackdowns by the administration of President Viktor Yanukovych.
After nightfall, the U.S. Embassy in Kyiv warned all Americans in the Ukrainian capital to remain indoors.
Public relations representatives for Ukraine's richest man, key Yanukovych backer Rinat Akhmetov, issued a statement on February 18 saying the Donetsk oligarch "strongly believes that there are no circumstances that would justify the use of force against peaceful citizens."
Opposition medics early in the day said three protesters had died of gunshot wounds and that another 150 were injured.
RFE/RL's Ukrainian Service has released video of two sets of two bodies lying on the ground.
In one, two bodies can be seen with their heads covered:
Kyiv emergency officials said one person was found dead at the offices of Yanukovych's ruling party. Earlier the same day, the building was stormed by protesters but police then pushed them away.
Protesters also attacked Kyiv city hall, which they had vacated just two days ago as part of a conditional amnesty for protesters detained during the antigovernment demonstrations.
Subway stations were closed virtually all day around Independence Square.
WATCH: Protesters force riot police to retreat on February 18.
Earlier on February 18, several thousand protesters had marched from that square to Ukraine's parliament to press for reforms to curb the president's powers.
The clashes erupted when protesters broke through a police cordon outside the parliament building.
The parliamentary session broke up in chaos after the speaker refused to put the opposition's reform initiatives on the daily agenda.
Opposition leader Vitali Klitschko called on Yanukovych to immediately call early presidential and parliamentary elections.
"I am appealing to the president of Ukraine, he bears all responsibility, he has all power in the country now," Klitschko said. "I am convinced that only he can solve this problem. That's why I'm appealing to him. As a responsible politician, as the president of Ukraine he should call early presidential and parliamentary elections. I'm convinced this will lower the temperature in the society. Do it and it will be a way out of the situation, this will be a brave decision. At the same time I'm urging the president to pull the riot police and Interior forces back off the streets."
A spokeswoman for President Barack Obama's National Security Council, Laura Lucas Magnuson, said the United States was "appalled by the violence taking place in downtown Kyiv" and urged the president to "immediately de-escalate the situation and end the confrontation at Maidan," as Independence Square is known.
The Russian Foreign Ministry blamed the latest escalation of tension on the "connivance" of Western politicians.
The head of the foreign affairs committee for the Russian State Duma, the lower house of parliament, Aleksei Pushkov said Ukraine was on the brink of civil war. Pushkov added that a "significant amount of responsibility for this falls on the West and Western politicians, who are constantly putting pressure on the Ukrainian authorities" and have offered support to antigovernment protesters.
High Representative of the European Union for Foreign Affairs and Security Policy Catherine Ashton issued a statement
warning of the danger facing Ukraine and its government.
"I am deeply worried about the grave new escalation in [Kyiv] and the reported victims," the EU's foreign policy chief said. "I condemn all use of violence, including against public or party buildings."
She recommended "the formation of a new inclusive government, progress on constitutional reform and the preparation for transpartent and democratic presidential elections. The EU stands ready to assist Ukraine in this process."
European Parliament President Martin Schulz said he was "shocked and saddened by reported killings."
Swedish Foreign Minister Carl Bildt issued an appeal for "a peaceful and non-violent solution" following news of the deaths during the clashes.
Polish Foreign Minister Radoslaw Sikorski called for "restraint and political compromise."
Antigovernment protests have been raging for nearly three months in Ukraine, with demonstrators occupying Independence Square and other buildings in a bid to oust Yanukovych.
On February 17, opposition leaders Arseniy Yatsenyuk and Klitschko met in Berlin with German Chancellor Angela Merkel to discuss Ukraine's political crisis.
Klitschko said after the meeting he urged the German chancellor to impose sanctions on Yanukovych
and his inner circle.
Also on February 17, Russian Finance Minister Anton Siluanov announced that Moscow will purchase $2 billion of Ukrainian eurobonds this week.
WATCH: RFE/RL's Ukrainian Service LIVE feeds from events in Kyiv earlier on February 18.
Kyiv and Moscow agreed in December that Russia would purchase $15 billion of Ukrainian eurobonds after Yanukovych pulled back from a planned free-trade agreement with the European Union, triggering the antigovernment protests.
Amid the ongoing political crisis, the national currency, the hryvnya, has lost 7 percent of its value this year.
Russian news agencies reported that the body of a Georgian national, reportedly born in 1960, was found in Kyiv on February 18. Georgian Ambassador to Ukraine Mikheil Ukleba confirmed the death of a Georgian, but the circumstances were unclear. At least one report quoted police as saying the Georgian's body bore no traces of violence.
With reporting by Reuters, AFP, Pravda.com.ua, and Bloomberg