Slovyansk Mayor Nelya Shtepa said there are some 1,000 people inside the city hall building and a Russian flag has been raised.
Police in the city say the pro-Russian separatists who seized the police station on the morning of April 12 have also taken control of some 400 handguns and 20 automatic weapons that were in the station.
Ukrainian Interior Minister Arsen Avakov wrote on Facebook early on April 12 that the men who took over the police station were wearing camouflage uniforms.
He said the "response will be very tough because there is a difference between protesters and terrorists."
Slovyansk is about 100 kilometers north of Donetsk and has a population of some 130,000 people.
Man strengthens a barricade in front of police HQ in Slovyansk (Eastern Ukraine) via @ajamlive pic.twitter.com/fg84IJKi8O
— Maxim Eristavi (@MaximEristavi) April 12, 2014
Reason for gunmen to seize #Slovyansk police station was to take the weapons from there - Interior Ministry statement http://t.co/ahm55Wbjm7
— Myroslava Petsa (@myroslavapetsa) April 12, 2014
Firearms from #Slovyansk police station are being handed out to the pro-Russian protesters - Interior Ministry
— Myroslava Petsa (@myroslavapetsa) April 12, 2014
Acting foreign minister Andrii Deshchytsia said he had spoken in a phone call with Russian Foreign Minister Sergei Lavrov.
Russia denies providing any support to the militants, who have seized four government buildings in the east of the former Soviet republic, apparently emboldened by Russia's annexation of Ukraine's Crimea region last month.
Talking to his Russian counterpart Sergei Lavrov via telephone on April 12, Deshchytsya said that the ongoing actions of Russian agents in eastern regions aim at disrupting the Geneva meeting on stabilization of the situation in Ukraine.
The Ukrainian foreign minister's comments come the same day an armed group in military uniforms seized a police station in the city of Slovyansk in Ukraine's eastern Donetsk region.
Pro-Russian armed groups have been occupying the regional administration building in Donetsk and Security Service headquarters in the city of Luhansk since April 6-7, demanding the federalization of Ukraine.
Deshchytsya also confirmed his participation in the U.S-EU-Russia-Ukraine talks in Geneva scheduled for April 17.
Похоже, в Украине либо будет кровавое воскресенье в некоторых городах, либо начнется второй этап распада страны, либо и то, и другое.
— Павел Шеремет (@pavelsheremet) April 12, 2014
MT @MaxRTucker: Pro-#Russia separatists have got themselves some up to date Russian automatic rifles #Ukraine pic.twitter.com/REsJCLlHZo
— Olga Tokariuk (@olgatokariuk) April 12, 2014
Vor der besetzen Polizeistation Slowjansk. Dutzende Bewaffnete und Vermummte #Donetsk #Ukraine pic.twitter.com/gpDL8hjpM7
— Dirk Emmerich (@DEmmerich) April 12, 2014
Crowd at police station in #donetsk @Ruptly pic.twitter.com/cs7eoGhkcF
— Monika Kalinowska (@mkalinowskaa) April 12, 2014