Throngs of protesters engaged in pitched battles with government security forces, who countered the demonstrators' rocks, bricks and wood projectiles with water cannons and stun grenades, CNN reported.
With the capital's central square ablaze with bonfires, the air filled with fireworks and smoke, and the death toll mounting, the Kiev Post said at least three armored vehicles had rolled into the Independence Square area, with demonstrators forcing one of them to stop.
The Post said at least 13 civilians and nine police officers had died, attributing the numbers to the Interior Ministry, physicians and opposition politicians. Dr. Olga Bogomolets told the Post the number of injured was more than 1,000 people and more likely "into the thousands."
President Viktor Yanukovych was to address the nation after an 11 p.m. meeting with his political opponents, including Vitali Klitschko, CNN reported.
At 4 p.m., acting Ukraine Security Services chief Oleksandr Yakymenko and acting Interior Minister Vitaliy Zakharchenko warned the protesters to clear the streets within 2 hours and if "the lawlessness doesn't cease, we shall be forced to used all legal means to bring order."
The Post said the state had taken over the state-owned exhibition hall and conference center by 6 p.m.
"For everyone who suffered, for every burned car and broken window, the organizers of the mass disturbances will carry responsibility," General Prosecutor Viktor Pshonka said in a statement. "The general prosecutor's office will demand the most severe punishment for those who pushed people into today's action, and those who organized and managed them."
Thousands of police officers encircled the central area teaming with protesters and assailed many of them, the Post said.
One protester with a head wound told the newspaper police officers "smashed everybody" in their path.
The Post said a police officer grabbed the gas mask of one of its reporters and said "I love it! We love it!" of the police action. The newspaper said police showed journalists bullet holes in their metal shields, saying demonstrators were responsible for them.
Oleh Tiahnybok, the leader of the opposition Svoboda Party, said it was a "fight for Ukraine's future" and called on citizens to join "our fight against this crazy regime."
City government officials warned people to stay away from the city center and the U.S. Embassy advised Americans "to maintain a low profile and remain indoors tonight."
U.S. Vice President Joe Biden spoke with Yanukovych by phone, expressing his "grave concern" about the situation.
"The vice president made clear that the United States condemns violence by any side, but that the government bears special responsibility to de-escalate the situation," the White House said in a statement.
U.S. Secretary of State John Kerry also condemned the violence.
"Rather than issuing ultimatums, the government of Ukraine should immediately resume talks with senior opposition leaders and support dialogue through Ukraine's democratic institutions, including parliament, the Rada," Kerry said. "We also call on protesters to refrain from violence of any kind; Ukraine's deep divisions will not be healed by allowing more innocent blood to be spilled."
The protest originally was called to coincide with Ukrainian lawmakers' consideration of changes to the country's constitution. But the Verkhovna Rada, as Ukraine's parliament is called, declined to take up the proposed changes, and police and protesters soon were in confrontations.
Geoffrey Pyatt, the U.S. ambassador to Ukraine, posted on his Twitter page, "We believe #Ukraine's crisis can still be solved via dialogue, but those on both sides who fuel violence will open themselves to sanctions."
Catherine Ashton, the top diplomat for the European Union, said in a statement: "I am deeply worried about the grave new escalation in Kiev and the reported victims. I condemn all use of violence, including against public or party buildings."
In Washington, White House spokesman Jay Carney said, "We are appalled by the violence that was already taking place in downtown Kiev and reports of armed riot police massing on the edge of Maidan [Independence Square]. We continue to condemn street violence and excessive use of force by either side. Force will not resolve the crisis."
"To restore, rather, peace and stability, we urge President [Viktor] Yanukovych to de-escalate immediately the situation and end the confrontation at Maidan," Carney said. "We also urge him to re-start a dialogue with opposition leaders today to develop a consensus way forward for Ukraine."
Protesters are seeking the restoration of Ukraine's 2004 constitution, which they said would solve the country's political crisis. The ruling Party of Regions said it would review a law reforming the constitution, but refused to return to a previous version, the RIA Novosti news agency reported.
Thousands of demonstrators have flocked to Kiev's Independence Square since November, when Yanukovych reversed a decision to sign a trade deal with the European Union and instead turned toward Russia.
An anti-protest law later went into effect, inflaming the protests as thousands of demonstrators filled the streets to protest the anti-free speech measures.
More than 2,000 people have been injured in the clashes.