Pro-Russian protesters occupying a government building in eastern Ukraine have declared the creation of a separatist republic.
The move has been described by Ukraine as part of a plan to justify a Russian invasion to dismember the country.
The Ukrainian government 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.
Russia annexed the Black Sea peninsula 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," he added.
Pro-Russian protesters seized official buildings in the eastern cities of Kharkiv, Luhansk and Donetsk last night.
They are demanding that referendums be held on whether to join Russia like the one that preceded Russia'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-Russia President Viktor Yanukovych, 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 to a cheering crowd of about 1,000 people outside the building.
Russian President Vladimir Putin announced on 1 March, a week after Mr Yanukovych was overthrown, that Moscow has the right to take military action in Ukraine to protect Russian speakers.
This has created 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.
However, much tougher measures have been threatened if Russian troops, now massed on the frontier, enter other parts of Ukraine.