Ukraine’s political split reflects a deeper cultural divide in the country. In the 2010 presidential election, the opposition won in all of Ukraine’s western provinces, where most people speak Ukrainian rather than Russian and many call for deeper economic and political ties with Europe.
But Vitali Klitschko, an opposition leader, expressed deep skepticism after his subsequent meeting with the European Union emissaries in Kiev. He was quoted by Agence France-Presse as saying there was “no deal yet.”
With demonstrators surging toward and then past police lines, what had been a narrowly circumscribed protest area ringed by police officers expanded rapidly and, amid a continual racket of gunshots, reached up a hill overlooking the square to the edge of the main government district of the capital.
The fighting left bodies lined up on a sidewalk and makeshift clinics crammed with the wounded, as sirens and gunfire rang through the center of the city.
“There will be many dead today,” Anatoly Volk, 38, one of the demonstrators, said as he watched victims on stretchers being carried down a stairway near the Ukraina Hotel.
Mr. Volk said the protesters had decided to try to retake the square because they believed the truce announced around midnight on Wednesday had been a ruse. The men in ski masks who led the push, he said, believed it was a stalling maneuver by Mr. Yanukovych to buy time to deploy troops in the capital because the authorities decided the civilian police had insufficient forces to clear the square.
“A truce means real negotiations,” Mr. Volk said. “They are just delaying to make time to bring in more troops. They didn’t have the forces to storm us last night. So we are expanding our barricades to where they were before. We are restoring what we had.”
Supporters of the opposition this week overran an Interior Ministry garrison near Lviv, in western Ukraine, and captured its armory. It was unclear whether any of the commandeered weapons were being used Thursday in the fighting in the capital.
The part of the square back under the control of the protesters after the fighting on Thursday was an otherworldly panorama of soot-smeared paving stones, debris and coils of smoldering wire from burned tires.
From the stage on the square, a speaker yelled “Glory to Ukraine!” and the crowd yelled back “Glory to its heroes!” That echoed the slogans of the World War II-era Organization of Ukrainian Nationalists and the Ukrainian Insurgent Army, guerrilla armies that battled the Nazis, Poles and Soviets in an ultimately futile quest for an independent Ukraine.
The protests began in November when Mr. Yanukovych rejected a trade and economic agreement with the European Union and turned instead to Russia for financial aid.
In a televised address to the nation on Wednesday, Mr. Yanukovych said opposition leaders had “crossed the limits when they called people to arms” and demanded that they “dissociate themselves from the radical forces that provoke bloodshed.”
The protest movement certainly contains extremist elements but, at least in Kiev and many other cities, particularly in the west, it has a wide base of public support. After talks with Mr. Yanukovych late Tuesday as violence spun out of control, the opposition leader Arseniy P. Yatsenyuk complained that the president had only a single offer: “that we surrender.”
Correction: February 20, 2014
An earlier version of a map with this article misstated the results of the 2010 election in a far western region of Ukraine. Viktor F. Yanukovych’s party did not win the region; the opposition did.