The coordinated attacks and a subsequent raid by a few hundred pro-Russian protesters on the police headquarters of the local capital Donetsk underscored the volatility of the crisis ahead of peace talks between EU and US diplomats and their Moscow and Kiev counterparts in Geneva on Thursday.
Ukraine's interim leaders have been facing unceasing pressure from Russia since their February ouster of an unpopular Kremlin-backed president and decision to seek closer ties with the West.
Moscow has massed tens of thousands of troops on Ukraine's eastern border after annexing its Black Sea peninsula of Crimea and nearly doubled the rates it charges Kiev for gas.
Russia is now ready to demand prepayment from the cash-strapped government for future gas deliveries or halt supplies -- a cutoff that would impact at least 18 EU countries and add further urgency to the worst East-West standoff since the Cold War.
But the seizures more immediately highlight how little sway Kiev's untested leaders have over pro-Russians who have since April 6 controlled the Donetsk government seat and a state security building in the nearby eastern city of Lugansk.
Ukrainian Prime Minister Arseniy Yatsenyuk visited the region yesterday in a failed bid to pacify protesters with a vague promise of more sweeping local rights.
The morning police station raid and a subsequent attack of the regional security service centre happened in Slavyansk -- a riverside town of 100,000 about 60 kilometres north of the regional capital Donetsk.
Ukraine's interior ministry said the first assault was led by 20 "armed men in camouflage fatigues" whose main purpose was to seize 20 machine guns, 400 Makarov guns stored in the police headquarters "and to distribute them to protesters."
"Our response will be very severe," Interior Minister Arsen Avakov wrote on his Facebook page.
"There is zero tolerance for armed terrorists."
An AFP reporter saw the Slavyansk police station being surrounded by armed men in masks and camouflage who had set up a barricade of old tyres and dumpsters in front of the police headquarters.
The gunmen aggressively shoved aside Western reporters and only allowed Russian-speaking media anywhere near the building.