Police battled their way into Independence Square, center of three months of protests against President Viktor Yanukovich, and demonstrators, some armed with clubs and wearing helmets and body armor, were losing ground.
Smoke billowed from burning tents and piles of tires and wood as thousands of protesters were pushed back on to the center of the square, a Reuters cameraman said.
The police was in control of almost half of the square and several floors of a trade union building, used as an anti-government headquarters, were on fire.
At least 14 protesters and seven policemen were killed in violence that erupted in Kiev on Tuesday and continued into the early hours of Wednesday. Many were killed by gunshot and hundreds more injured, with dozens of them in serious condition, police and opposition representatives said.
Alarmed Western governments demanded restraint and dialogue. U.S. Vice President Joe Biden called Yanukovich, urging him to pull back government forces and exercise maximum restraint, the White House said.
Opposition leaders Vitaly Klitschko and Arseny Yatsenyuk said that they had quit talks with President Viktor Yanukovich without reaching any agreement on how to end the violence.
“The government must immediately withdraw troops and put an end to the bloody conflict, because people are dying. I told Yanukovich this,” Klitschko said after the late night talks. “How can we hold talks while blood is being shed?”
The unrest has spread to at least three cities in the western part of the country. Police said protesters had seized regional administration headquarters in the cities of Ivano-Frankivsk and Lviv. Local media said protesters torched the main police station in the city of Ternopil.
The authorities have restricted traffic coming into the capital to prevent protesters from adding reinforcements. Kiev authorities also closed down the underground.
Earlier, the state security service set a deadline for the demonstrators to end disorder or face “tough measures”. Then the police advanced to the square before launching a full assault in the early hours, throwing stun grenades and using water cannons.
Nationwide demonstrations erupted in November after Yanukovich bowed to Russian pressure and pulled out of a planned far-reaching trade pact with the European Union, deciding instead to accept a Kremlin bailout for the heavily indebted economy.
Western powers warned Yanukovich against trying to smash the pro-European demonstrations, urging him to turn back to the EU and the prospect of an IMF-supported economic recovery, while Russia accused them of meddling.
Ukraine has been rocked periodically by political turmoil since gaining independence from the Soviet Union more than 22 years ago, but it had never experienced violence on this scale.
“We are now facing of one of the most dramatic episodes in Ukrainian history,” opposition leader Yatsenyuk said in a video message after emergency talks with the president failed.
As the security forces moved forward, Klitschko, a former world champion boxer, reacted defiantly, telling supporters on the square: “We will not leave here. This is an island of freedom. We will defend it.”
Earlier on Tuesday the State Security Service (SBU), in a joint statement with the Interior Ministry, signaled the government’s intentions. “If by 6 p.m. the disturbances have not ended, we will be obliged to restore order by all means envisaged by law,” they said.
The riot police moved in hours after Moscow gave Ukraine $2 billion in aid for its crippled economy that it had been holding back to demand decisive action to crush the protests.