The clashes outside parliament erupted after the opposition accused the government of ignoring its demands even after nearly three months of protests that have paralyzed the capital. It was the worst violence since the protests began in late November.
As darkness fell, opposition leaders warned that security forces may be preparing to clear a sprawling protest tent camp on Kiev's Independence Square. Law enforcement agencies vowed to bring order to the streets and shut down subway stations in the capital.
Thousands of protesters streamed to the square to defend the camp, where Orthodox priests prayed for peace.
"We see that this regime again has begun shooting people; they want to sink Ukraine in blood. We will not give in to a single provocation," opposition leader Arseniy Yatsenyuk told the protesters.
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"We will not take one step back from this square. We have nowhere to retreat to. Ukraine is behind us, Ukraine's future is behind us."
Olha Bilyk, spokeswoman for the Kiev city police, told The Associated Press that two policemen were killed, likely by gunshot wounds, in today's clashes and seven civilians died, including three who were shot.
Riot police retaliated with stun grenades and fired what appeared to be small metal balls, as smoke from burning tires and vehicles billowed over Kiev.
The clashes dimmed hopes for an imminent solution to the political crisis - and tensions also soared following new steps by Russia and the European Union to gain influence over this former Soviet republic.