Correction: Earlier versions of this article said that, according to medics, a demonstrator died after falling off a bridge. Medics said later that the person survived the fall.

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 encamped 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 cronyism in 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 they could be seen aiming rifles during the day. The dead are being portrayed as martyrs.

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

“If the president does not go forward” on 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, which has been in place since Nov. 21.

They had twice swept away the young protesters 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 a bullet in my head, then so be it,” he said.

“We’re going to kick the authorities out. We’re going to win and have the new Ukraine that we deserve,” said Oleh Tiahnybok, head of the nationalist Svoboda party, who also took part.

A galvanized public

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.

Youths 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 last month. Rain and warm weather have since caused considerable depletion, but the cold has returned, and that’s welcome news for the opposition.

On Hrushevsky, protesters set alight piles of tires, sending thick black smoke westward across Kiev. Their goal was to create a smokescreen behind which they industriously constructed a fallback barricade in an attempt to prevent another police sweep.

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.

The activist beaten to death was Yuriy Verbytskiy, a resident of the western city of Lviv who was on Hrushevsky Street. He was kidnapped from a Kiev hospital Tuesday night after he had sought treatment for a splinter in his eye. Another man, Ihor Lutsenko, who was abducted with him, was beaten but later released. Verbytskiy’s body was found near the airport, according to his brother.

The protest began when Yanukovych suddenly backed off a trade deal with Europe and turned to Russia for support. It quickly broadened to include demands that he step down. Throughout, protest leaders continually stressed the need to use peaceful means to pursue their ends.

But Yanukovych hung on, and last week his party rammed through harsh new laws restricting protest and speech. On Sunday evening, a breakaway group of frustrated young nationalists, led by a group called Pravy Sektor, marched off the Maidan to Hrushevsky Street, where they were confronted by police and began throwing stones, fireworks and molotov cocktails at them. Police responded with stun grenades and rubber bullets.

The confrontation on Hrushevsky Street persisted, and it complicated the task for opposition leaders who have denounced violence as self-defeating. But the sight of young men taking the fight to the police thrilled thousands of spectators who didn’t play a direct role in the clashes.

U.S. ‘condemns’ violence

The violence, and especially Wednesday’s escalation, seemed only to harden the resolve of both sides, even as the talks were getting underway. Prime Minister Mykola Azarov said the opposition has to decide once and for all whether it will renounce the violence of the nationalists on Hrushevsky or embrace it and suffer take the consequences. He then flew off to an economic forum in Davos, Switzerland — though news agencies here reported that his invitation had been withdrawn.

The U.S. Embassy in Kiev announced that it has revoked the visas of Ukrainians it deems responsible for the violence here. It said it would not identify those people, but official U.S. statements on the crisis suggest that those affected are probably Ukrainian government or police officials.

In Washington, Marie Harf, a spokeswoman for the State Department, released a statement saying that the United States “strongly condemns” the violence in Kiev.

“Increased tensions in Ukraine are a direct consequence of the Ukrainian government’s failure to engage in real dialogue and the passage of anti-democratic legislation on January 16,” it said. “However, the aggressive actions of members of extreme-right group Pravy Sektor are not acceptable and are inflaming conditions on the streets and undermining the efforts of peaceful protesters. We likewise deplore violence by unofficial groups known as ‘titushki.’

“We also condemn the targeted attacks against journalists and peaceful protesters, including detentions.”

The issue of thugs for hire, working on behalf of Yanukovych and known here as “titushki,” has become a potent addition to the grievances of the protesters.

Opposition groups have been scouting the streets of Kiev the past two nights in search of titushki, who are accused of smashing cars and beating up passersby. At least a dozen young men have been rounded up and taken to the protest headquarters. Almost all of them are teenagers.

The Interior Ministry declared that 40 people were “kidnapped” in Kiev on Tuesday night, clearly referring to what the opposition believes is an act of self-defense.