MOSCOW—Pro-Russia protests spread Saturday in eastern Ukraine as armed men, some in unmarked, military-style uniforms, moved to commandeer more government buildings—a dramatic escalation that the Ukrainian government and Washington tied to Moscow.

Ukraine's acting president called an emergency meeting of the country's top security officials in the capital, Kiev, as police clashed with attackers in several spots in the largely Russian-speaking, eastern part of the country.

Some 20 men wearing camouflage, some with military-style equipment and weapons, commandeered a police station and security-service office early Saturday in Slavyansk, a small city that previously hadn't seen the kind of pro-Russian fervor that erupted anew in the area last weekend.

Ukraine's Interior Minister Arsen Avakov said Saturday evening that police were fighting with attackers in the nearby town of Kramatorsk and had repelled a similar attack in a third town, Krasny Liman.

He wrote on Facebook that the moves amounted to "aggression by the Russian Federation," noting that some of the weapons that were used in the assaults were available only to the Russian military.