* Three government buildings now seized by pro-Moscow militants
* Pro-Western Kiev government promises tough response
* Protesters briefly hold part of Donetsk prosecutors office (Adds police in talks with attackers, new seizure in Donetsk)
By Gleb Garanich
SLAVIANSK, Ukraine, April 12 (Reuters) - At least 15 armed men seized police headquarters in the eastern Ukrainian city of Slaviansk on Saturday, extending takeovers of public building by pro-Russian militants who have been demanding autonomy from Kiev.
Government buildings in two other Russian-speaking cities, Donetsk and Luhansk, have been occupied by separatists since last weekend, in what the new pro-Europe leadership in Kiev says is part of a plan drawn up by the Kremlin to dismember Ukraine.
Russia annexed Ukraine's Russian-majority Crimea region last month after pro-Europe protesters toppled Kiev's Kremlin-backed president. Moscow denies having any designs on other regions of Ukraine but says Russian speakers there must be protected from possible persecution by the interim authorities in Kiev.
"Armed men in camouflage uniforms seized the police department in Slaviansk. The response will be very tough because there is a difference between protesters and terrorists," Interior Minister Arsen Avakov wrote on his Facebook.
A local police spokesman said it estimated 15-20 armed men had taken over the building in Slaviansk, in Ukraine's Donetsk region, about 150 km (93 miles) from the border with Russia.
Police were talking to the attackers, but they had not yet made any specific demands, he said.
Masked men in mismatched camouflage clothes armed with pistols and rifles stood near the building, a Reuters witness said. They were wearing orange and black ribbons, a symbol of the Soviet victory in World War Two that has been adopted by pro-Russian separatists in Ukraine.
A few hundred people gathered outside the three-storey building in a residential district near the centre of Slaviansk, a city of more than 100,000 inhabitants. Some in the crowd helped to build barricades from tires.
The militants in the Donetsk local government building and a security services building in Luhansk are pressing for a referendum on the future of the region.
The Ukrainian government says the takeovers could be part of a plan similar to that used by Russia to annex Crimea - the seizure of government buildings and military facilities was followed by a referendum on independence.
"I will say it again: those who want dialogue... will have dialogue and the search for solutions. Those who are up in arms, set fire to buildings, shoot at people, police, terrorise with bats and masks, these people will face an appropriate response," Avakov said in his Facebook post.
On Friday, a deadline set by the Kiev authorities for the protesters to end their occupations expired, but there was no sign of action from the Ukrainian police to force them out.
In Donetsk on Saturday, a group of around 40 young people armed with wooden bats briefly took over a floor of the general prosecutors office, barricading themselves in with furniture.
The protesters later agreed to leave following negotiations, Donetsk police said in a statement. (Additional reporting by Pavel Polityuk and Lina Kushch; Writing by Conor Humphries; Editing by Mark Heinrich)