* Yatseniuk says protests are part of script written in Russia
* Ukraine will "not allow" Russian troops across the border
* Says Ukraine have "clear action plan" to handle crisis (Adds details)
By Richard Balmforth and Lina Kushch
KIEV/DONETSK, Ukraine, April 7 (Reuters) - Pro-Moscow protesters occupying a government building in eastern Ukraine declared the creation of a separatist republic on Monday, in a move Kiev described as part of a plan to justify a Russian invasion to dismember the country.
Kiev said the seizures of state buildings in three cities in eastern Ukraine's mainly Russian speaking industrial heartland were a replay of events in Crimea, the Black Sea peninsula Moscow annexed after its troops occupied it last month.
"An anti-Ukrainian plan is being put into operation ... under which foreign troops will cross the border and seize the territory of the country," Prime Minister Arseny Yatseniuk told a cabinet meeting in Kiev. "We will not allow this."
Pro-Russian protesters seized official buildings in the eastern cities of Kharkiv, Luhansk and Donetsk on Sunday night, demanding that referendums be held on whether to join Russia like the one that preceded Moscow's takeover of Crimea.
Police said they cleared the protesters from the building in Kharkiv, but in Luhansk the demonstrators had seized weapons.
In Donetsk, home base of deposed pro-Russian President Viktor Yanukovich, about 120 pro-Moscow activists calling themselves the "Republican People's Soviet of Donetsk" seized the chamber of the regional parliament.
An unidentified bearded man read out "the act of the proclamation of an independent state, Donetsk People's Republic" in front of a Russian flag.
"In the event of aggressive action from the illegitimate Kiev authorities, we will appeal to the Russian Federation to bring in a peacekeeping contingent," said the proclamation.
The activists later read out the text by loud hailer to a cheering crowd of about 1,000 people outside the building.
Russian President Vladimir Putin announced on March 1, a week after Yanukovich was overthrown, that Moscow has the right to take military action in Ukraine to protect Russian speakers, creating the biggest confrontation between Moscow and the West since the Cold War.
The United States and EU imposed mild financial sanctions on a limited number of Russian officials over the seizure of Crimea but have threatened much tougher measures if Russian troops, now massed on the frontier, enter other parts of Ukraine.
Interior Minister Arsen Avakov said on Monday the main regional administration building in Kharkiv had been cleared of "separatists". But police in Luhansk said protesters occupying the state security building there had seized weapons. Highway police had closed off roads into the city.
"Unknown people who are in the building have broken into the building's arsenal and have seized weapons," a police statement said. Nine people had been hurt in the disturbances in Luhansk.
Mainly Russian-speaking eastern Ukraine has seen a sharp rise in tension since Yanukovich fled the country, and Kiev has long said it believes Moscow is behind the unrest.
Unlike in Crimea, where ethnic Russians form a majority, most people in the east and south are ethnically Ukrainian but speak Russian as a first language. Influential businessmen in eastern regions who once supported Yanukovich have mostly thrown their weight behind the government in Kiev, and the unrest there is a test of their ability to assert their control.
Yanukovich, in exile in Russia, has called for referendums across Ukrainian regions on their status within the country.
Ukraine's defence ministry said a Russian marine had shot and killed a Ukrainian naval officer in Crimea on Sunday night. The 33-year-old officer, who was preparing to leave Crimea, was shot twice in officers' quarters. It was not clear why the Russian marine had opened fire. Ukraine has declared that it is pulling its troops out of Crimea after Russian forces seized it.
Yatseniuk said that though much of the unrest had died down in eastern Ukraine in the past month there remained about 1,500 "radicals" in each region who spoke with "clear Russian accents" and whose activity was being coordinated through foreign intelligence services.
But he said Ukrainian authorities had drawn up a plan to handle the crisis.
"We have a clear action plan," he said, adding that senior officials would head to the towns concerned.
Avakov on Sunday accused Russian President Vladimir Putin of orchestrating the "separatist disorder" and promised that disturbances would be brought under control without violence.
Russia has been pushing internationally a plan proposing the "federalisation" of Ukraine in which regions of the country of 46 million would have broad powers of autonomy.
Ukraine, while drawing up its own blueprint of constitutional changes for "de-centralisation", in which smaller municipalities would be able to develop their own areas by retaining a portion of state taxes raised, says the Russian plan is aimed at breaking up the country.
Referring to the Russian plan, Yatseniuk said: "It is an attempt to destroy Ukrainian statehood, a script which has been written in the Russian Federation, the aim of which is to divide and destroy Ukraine and turn part of Ukraine into a slave territory under the dictatorship of Russia," he said.
"This is not going to happen," he said.
"I appeal to the people and the elites of the east. Our common responsibility is to preserve the country and I am sure that no-one wants to be under a neighbouring country. We have our country. Let's keep it," he said. (Additional reporting by Thomas Grove, Natalia Zinets and Pavel Polityuk; Writing by Richard Balmforth and Peter Graff)