KIEV, Ukraine—At least two protesters died of gunshot wounds as police cracked down on antigovernment demonstrations, marking the first deaths and a dramatic escalation in the conflict over whether this former Soviet republic will move closer to the West or Russia.

As President Viktor Yanukovych held talks with the leaders of the three main opposition parties Wednesday that yielded no immediate results, the weekslong confrontation spiraled into some of the most violent clashes seen on the streets of Kiev since the protests began two months ago.

Protesters were girding for assaults overnight as authorities appeared to prepare for a showdown, and opposition leaders threatened to go on the offensive. Riot police moved in with water cannons and, for the first time, at least one armored personnel carrier.

The West heavily criticized the violence and deaths. The U.S. said Wednesday that it had revoked visas for several Ukrainians involved in violence against protesters late last year. State Department officials called the increased tensions a "direct consequence" of the government's crackdown and warned of the possibility of further, targeted sanctions against Ukrainians who order more violence.

Mr. Yanukovych showed little indication of backing down in the face of the tough rhetoric. Russia, which has backed Mr. Yanukovych since he shelved an integration pact with the European Union last fall in favor of closer ties with Ukraine's former Soviet master, has endorsed the clampdown on protesters. On Wednesday, Moscow criticized the West for interfering.