KIEV - Separatist protesters on Sunday seized control of the mayor's office in the town of Mariupol, eastern Ukraine, on the Azov Sea, local media said.
The protesters entered the building following a rally involving about 1,000 people demonstrating in favour of the creation of a separate republic in eastern Ukraine, a local journalist for the newspaper Priazovsky Worker said.
The police did not try to stop them. They entered the offices, took down the Ukrainian flag and were erecting barricades outside the building, the journalist said.
Ukrainian security forces launched an operation on Sunday to clear pro-Russian separatists from a police headquarters in the eastern city of Slaviansk, with Kiev reporting dead on both sides as it combats what it calls an act of aggression by Moscow.
Ukraine faces a rash of rebellions in the east which it says are inspired and directed by the Kremlin. But action to dislodge the armed militants risks tipping the stand-off into a new, dangerous phase as Moscow has warned it will protect the region's Russian-speakers if they come under attack.
One Ukrainian state security officer was killed and five wounded on the government side in what Interior Minister Arsen Avakov called Sunday's "anti-terrorist" operation.
"There were dead and wounded on both sides," Mr Avakov said on his Facebook page, adding that there had been an "unidentifiable number" of casualties among the separatists, who were being supported by about 1,000 people.
Kiev accuses Moscow of trying to deepen violence and chaos in Ukraine, a former Soviet republic it once ruled. The Kremlin, it says, wants to undermine the legitimacy of presidential elections on May 25 which aim to set the country back onto a normal path after months of turmoil.
Relations between Russia and the West are at their worst since the end of the Cold War, the result of the crisis that began when the Moscow-backed Ukrainian president was pushed out and Russia annexed Crimea from Ukraine.
Moscow justified sending its military into Crimea, on Ukraine's southern tip, by saying the Russian population there was under threat, and some in Western governments believe the Kremlin is preparing a similar scenario for eastern Ukraine.
A Reuters reporter in Slaviansk, about 150 km from the Ukraine-Russia border, said two military helicopters were flying over the town's police headquarters where the militants were holed up.
Earlier, Mr Avakov had warned residents to stay indoors. "Pass this on to all civilians: they should leave the centre of town, not come out of their apartments, and not go near the windows," he wrote on his Facebook page.