Arseniy Yatsenyuk, a former foreign minister who led efforts to bring Ukraine closer to the European Union, told a large crowd on Kiev's central square that President Viktor Yanukovych must still meet several key opposition demands and that talks will continue.
The overnight outburst underlined a growing inclination for radical actions among some in the protest movement that has gripped the capital for two months.
In the dark of night, demonstrators threw firebombs into the Ukrainian House building and setting off fireworks, and police responded with tear gas. Although the crowd created a corridor at the building's entrance apparently for police to leave, none were seen coming out.
The building under attack is about 250 meters (yards) down the street from Independence Square, where mostly peaceful demonstrations have been held around the clock since early December and where protesters have set up an extensive tent camp.
More From This Section
The protests began in Kiev after Yanukovych shelved a long-awaited trade pact with the EU in favor of securing a bailout loan from Russia, and boiled over into violence a week ago over new anti-protest laws.
Yanukovych's latest offer, coming as protester anger rises and spreads from the capital to a wide swath of the country, appeared to have been both a concession and an adroit strategy to put the opposition in a bind.
Accepting the offer could have tarred Yatsenyuk among protesters as a sell-out, but rejecting it could make him appear obdurate and unwilling to seek a way out of the crisis short of getting everything the opposition wants. Yanukovych also offered Klitschko a deputy premiership.