KIEV, Ukraine — With President Viktor F. Yanukovych and antigovernment demonstrators at an impasse here, a cease-fire disintegrated on Friday night as the Ukrainian capital convulsed in renewed violence and fire bombs lit up the night sky.

Civil unrest spread across the country earlier on Friday as protesters laid siege to government buildings in at least nine other cities — occupying some and thronging outside others.

The widening turmoil, in the central Ukrainian cities of Khmelnitsky, Zhytomyr and Cherkasy, as well as in the western strongholds of Lviv, Ternopil, Ivano-Frankivsk, Lutsk, Rivne and Chernivtsi, showed that the authorities, including the elite Berkut riot police and Interior Ministry troops, were outnumbered and at risk of being spread too thin.