Officials in Ukraine say armed men dressed in camouflage clothing have seized a police station in the east of the country.
Police say the gang fired shots and used stun grenades to seize the offices in Sloviansk, near the Russian border.
Interior Minister Arsen Avakov called the gunmen "terrorists" and said special forces would repel the attack.
Pro-Russian protesters have taken over government buildings throughout eastern Ukraine. Kiev says the unrest is being orchestrated from Moscow.
Protesters in the eastern city of Donetsk, 130km from Sloviansk, have been occupying government buildings for days and are demanding a referendum on becoming part of Russia.
A similar move prompted a Russian takeover of Ukraine's Crimea region earlier this year.
The United States and the European Union have imposed sanctions on Russian and Crimean people they say were connected with the takeover.
Russia has denied responsibility for the protests in eastern Ukraine, but Western nations have expressed concern over a build-up of Russian troops along the border.
Interim Prime Minister Arseniy Yatsenyuk visited Donetsk on Friday and attempted to placate anti-government groups by guaranteeing that no restrictions would be put on the use of the Russian language.