KIEV/DONETSK, Ukraine (Reuters) - Pro-Moscow protesters in eastern Ukraine seized arms in one city and declared a separatist republic in another, in moves Kiev described on Monday as part of a Russian-orchestrated plan to justify an invasion to dismember the country.
Kiev said the overnight seizure of public 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 seized and annexed 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 said in public remarks to his cabinet. "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.
Acting President Oleksander Turchinov, in a televised address to the nation, said Moscow was attempting to repeat "the Crimea scenario". He added that "anti-terrorist measures" would be deployed against those who had taken up arms.
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 Moscow-backed President Viktor Yanukovich, about 120 pro-Russia activists calling themselves the "Republican People's Soviet of Donetsk" seized the chamber of the regional assembly.
An unidentified bearded man read out "the act of the proclamation of an independent state, Donetsk People's Republic" in front of a white, blue and red 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," ran 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 had 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 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.
Western European governments have hesitated to alienate Russia further, fearing for supplies of Russian natural gas, much of which reaches EU buyers via pipelines across Ukraine.
In Vienna, Russia did not attend a meeting on Ukraine of the Organisation for Security and Cooperation in Europe. The U.S. envoy to the OSCE, Daniel Baer, said tens of thousands of Russian troops were massed on the Ukrainian border, and any call by pro-Moscow activists for them to intervene was "tightly coordinated with the Russian government".
Ukrainian 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.
"Unidentified people who are in the building have broken into the building's arsenal and have seized weapons," police said in a statement. Nine people had been hurt in the disturbances in Luhansk.
Mainly Russian-speaking eastern Ukraine, densely populated and producing much of the country's industrial output, has seen a sharp rise in tension since Yanukovich fled the country, and Kiev has long said it believes Moscow is behind the unrest.
Pro-Russian protesters briefly held public buildings in the east early last month and three people were killed in clashes in mid-March. But trouble had subsided until Sunday.
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 business owners 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 abroad. 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 accused Putin on Sunday 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 plan for "de-centralisation", in which smaller municipalities would be able to retain a portion of state taxes, says the Russian proposal is aimed at breaking up the former Soviet republic.
"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," Yatseniuk said of what called the Russian plan.
"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," he said.
"We have our country. Let's keep it."