KIEV, Ukraine — With at least two protesters dead from gunshot wounds, and another found beaten to death, talks that opened Wednesday afternoon between President Viktor Yanukovych and the three main leaders of the political opposition are likely to be the last chance to head off a nation-shaking physical confrontation.

Aggressive riot police had put the center of Kiev, with its many thousands of protesters, in a state of considerable tension. Sporadic conflict continued all day outside Dynamo Stadium as protesters, led by a right-wing splinter group, refused to give ground to Interior Ministry troops.

At stake is the future of this country, torn between Russia and Europe, and riven by anger over the corruption and brutality of Yanukovych’s government. For two months, a partnership of pro-Europe liberals, die-hard nationalists, and critics of the ruling family and its circle of oligarchs has carried out a protracted campaign of protest, almost all of it peaceful. That moment could be nearing an end.

The deaths of three and possibly more activists shocked and energized the legions of protesters, who again turned out Wednesday evening. One of the victims was shot four times, medics said.

Police denied they had used firearms, though during the day they could be seen aiming rifles. The dead are being portrayed as martyrs to the Yanukovych regime.

The politicians broke off after three hours, and the opposition leaders later delivered angry avowals of their determination to oust Yanukovych but said they planned to meet again Thursday.

“If the president does not go forward” toward a peaceful resolution, said Vitali Klitschko, head of the opposition UDAR party, “tomorrow we’ll go on the offensive.”

As darkness fell, tens of thousands of protesters flocked through snow to be on hand to defend Independence Square, known to all simply as the Maidan, after fears spread that the police would try Wednesday night to clear the encampment that has persisted since Nov. 21.

They had already twice swept away the young men on nearby Hrushevsky Street, by the stadium, but the opposition forces reclaimed the street each time.

The aggressive police action occurred on Unification Day, a national holiday. It drove the already deep divide between Yanukovych and his opponents to the point where a negotiated settlement looks difficult.

The two sides have just 24 hours to prevent bloodshed, Arseniy Yatsenyuk, of the opposition Fatherland party, said Wednesday night after he left the president’s office. “After that, if I get bullet in my head, then so be it,” he said.

The mood on the Maidan on Wednesday evening, as it filled with protesters, was serious but not grim. As young men in helmets and improvised body armor came and went from Hrushevsky Street, old women on the Maidan watched and made the sign of the cross.

Younger people furiously chipped away at the newly fallen but already packed snow, while others scooped it up into sandbags, to be used to bolster the formidable barricades built on foundations of ice and snow in December.

The police denied that they had fired on the protesters, though they could be clearly seen aiming rifles during the day.

Many of those injured Wednesday were broadcast journalists and medics, leading to accusations that they had been deliberately targeted.

All told, hundreds of injuries have been reported, on both sides.