Kostyantynivka has 80,000 inhabitants and is located mid-way between the flashpoint town of Slavyansk and the regional hub city of Donetsk, both of which are also under the control of insurgents.
Some 20 militants armed with Kalashnikov assault rifles and wearing uniforms with no insignias took control of the town hall and hoisted the flag of the "Donetsk Republic".
The gunmen also set up a perimeter around the adjoining police station.
They used the same tactics and weapons as those seen during in the lead-up to the annexation of Crimea which Moscow later admitted involved the deployment of Russian military forces.