* Gunmen seize local police HQ, administration building
* Demand referendum, threaten independence (Updates with quotes, details)
By Maria Tsvetkova
KOSTYANTYNIVKA, Ukraine, April 28 (Reuters) - Armed pro-Russian separatists in masks and military fatigues seized the police headquarters and state administration building in the east Ukrainian town of Kostyantynivka on Monday, as an uprising against Kiev appeared to spread.
Reuters reporters at the scene said each building was under the control of around 20 well-organised gunmen, some in flak jackets and carrying automatic weapons. They erected a barricade of tyres, sandbags and concrete blocks unloaded from a lorry in front of the administration building.
Soviet songs played over loud-speakers as women gathered the signatures of people wanting to join the uprising.
An industrial town in the eastern Donetsk region, Kostyantynivka is near the towns of Kramatorsk and Slaviansk, where armed separatists have clashed with Ukrainian security forces. Separatists in Donetsk have proclaimed an independent "People's Republic", threatening to break away following Russia's annexation of Crimea in March.
"I think it's the usual demands, the same as in other towns, the scenario doesn't change: a referendum and a federal system of government," said Ilya Suzdaliev, a spokesman for the official Donetsk regional administration, which includes Kostyantynivka.
One of the gunmen, who like others refused to give his name, said: "We have taken the (administration) building under our guard. We want to conduct a referendum."
At the police headquarters, some police officers remained and police vehicles were seen entering and leaving the grounds.
One of the armed men said he was from the Donetsk region. Another spoke in Ukrainian, and when asked why they had taken over the buildings, he replied: "My motherland ordered it."
Asked which motherland, he used a term which means new Russia, saying: "Novorossiya".
"The southeast of Ukraine, that's Novorossiya," he said. "The Donetsk republic is just temporary, then it will be the Novorossiya Republic, either independent or as part of Russia."
(Additional reporting by Natalia Zinets in Kiev and Marko Djurica in Kostyantynivka, Ukraine; Writing by Matt Robinson, Editing by Timothy Heritage)
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