Ukrainian President Viktor Yushchenko has threatened to dissolve parliament and call elections after the collapse of the country's ruling coalition.
He said a "coup" was under way in the country's parliament, following new laws trimming the president's powers.
The laws were introduced by the pro-Russian opposition and a bloc headed by Prime Minister Yulia Tymoshenko.
Former allies, the prime minister and president are now at odds despite sharing pro-Western political goals.
"A political and constitutional coup d'etat has started in the parliament," Mr Yushchenko said in a televised speech.
"I will use my right to dissolve parliament and decree early elections if a new Georgia breaks ties with Russia ...
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'Irresponsible behaviour'
The BBC's Russian affairs analyst Steven Eke says Mr Yushchenko's popularity is at rock bottom at the moment with opinion polls giving him single-digit levels of support.
He says his chances of re-election in any presidential vote would be slim.
Ms Tymoshenko said the Ukrainian cabinet would continue its work despite the break-up of the coalition.
"I am sorry that the president behaves irresponsibly," she said at a cabinet meeting. The coalition was destroyed under his instruction."
The crisis follows mounting tension between the president and prime minister with Mr Yushchenko accusing Ms Tymoshenko of treason for allegedly siding with Moscow over the conflict in Georgia.
The flare-up comes a day before a planned visit to the country by US Vice-President Dick Cheney.
He is touring former Soviet states to the south of Russia which the US sees as key allies.
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(BBC)
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