Lines of riot police surrounded the back of the building, facing hundreds of men and women. At the front, dozens of men, some in green camouflage and holding shields, walked into the imposing, white building, while others smashed windows and raised the Russian tricolour.
The government in Kiev has all but lost control of its police forces in parts of eastern Ukraine since pro-Russian activists seized buildings in the region's second biggest city of Donetsk and several smaller towns.
In separatist-held Slaviansk, the self-declared mayor said he would discuss the release of detained military observers only if the European Union dropped sanctions against rebel leaders.
"The regional leadership does not control its police force," said Stanislav Rechynsky, an aide to Interior Minister Arsen Avakov, referring to events in Luhansk where separatists had earlier only occupied the local security services' building.
"The local police did nothing," he said, adding that the ministry had information that they would next try to take the local television centre.
Some protesters, who say they are not properly represented in Kiev by Ukraine's new pro-Western rulers, called on the police to hand over their arms.
Ukraine's authorities are struggling to find a way to evict the separatists, who also took a small town hall in Pervomaisk in the Luhansk region yesterday.
Kiev launched an "anti-terrorist" operation in early April, but so far it has failed to yield many results.
Some separatists have begun to call the shots.
Vyacheslav Ponomaryov, the self-declared mayor of Slaviansk, told Interfax news agency that EU sanctions against two rebel leaders were "not conducive to dialogue" on releasing six military observers detained after being accused of harbouring a Ukrainian spy.
The observers had travelled to eastern Ukraine under the auspices of the Organisation for Security and Co-operation in Europe (OSCE), a democracy watchdog.
"If they fail to remove the sanctions, then we will block access for EU representatives, and they won't be able to get to us. I will remind my guests from the OSCE about this," he said.
Separatists in Donetsk, eastern Ukraine's second biggest city, have said they will hold a referendum on independence for the Donbass region on May 11.
That would undermine government efforts to hold a non-binding consultative referendum across Ukraine on May 25 or June 15, when the country votes for the president, to gauge appetite for the decentralisation of power.
Russia has denied suggestions that it is orchestrating events in eastern Ukraine and that it plans to invade eastern Ukraine, which is home to many Russian speakers, following its annexation of the Crimea peninsula.
Meanwhile, the European Union announced asset freezes and travel bans on 15 more Russians and Ukrainians over Moscow's actions in Ukraine, but the measures were seen as less aggressive than sanctions imposed this week by the US.