Mr Yushchenko said he would call a poll unless a new coalition was formed
Ukrainian President Viktor Yushchenko has threatened to dissolve parliament and call elections after the collapse of the country's ruling coalition.
Mr Yushchenko's supporters walked out in protest on Tuesday night following new laws to trim presidential powers.
The laws were introduced by the opposition and backed by Prime Minister Yulia Tymoshenko's bloc.
Former allies, the prime minister and president are now at odds despite sharing pro-Western political goals.
All but one of 12 ministers from Mr Yushchenko's party boycotted Wednesday's cabinet meeting.
"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 coalition is not formed within 30 days," he said.
Under Ukrainian law, parliament has 30 days to form a new coalition after one is dissolved, and another month to put together a cabinet. Until then, the previous government continues its work.
Analysts said that with the current coalition still legally in force, Mr Yushchenko had no formal authority to push for a new alliance.
Ms Tymoshenko blamed her rival for the chaos, vowing that the Ukrainian cabinet would continue its work despite the break-up of the coalition.
"Bloc Yulia Tymoshenko declares that there is no alternative to the democratic coalition," her party said on its website.
In a televised address, Ms Tymoshenko said: "I am sorry that the president behaves irresponsibly. The coalition was destroyed under his instruction."
"I am convinced that the work of the democratic coalition will be renewed. You have 10 days... to return to the democratic coalition," she added.
The BBC's Gabriel Gatehouse, in Kiev, says the coalition has been weak since it was formed and has been teetering on the brink of collapse for some time.
This move is seen by many as the first shots in a presidential campaign in which Mr Yushchenko and Ms Tymoshenko are both candidates, our correspondent says.
Polls have shown that Ms Tymoshenko would win a presidential election, with Yushchenko coming last. In a parliamentary election, she would increase her seats, while Mr Yushchenko's Our Ukraine party would lose some.
If parliamentary elections are called, they would be the third in as many years since the 2004 "Orange Revolution", which swept Mr Yushchenko to power.
He campaigned on a ticket of greater Western integration including joining Nato and the European Union.
The BBC's regional 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.
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.
Mr Yushchenko has been a vociferous supporter of Georgia during the conflict but the prime minister's party on Tuesday blocked a parliamentary resolution condemning Moscow.
The flare-up comes a day before a planned visit to the country by US Vice-President Dick Cheney.
The trip is part of a tour of former Soviet states which the US sees as key allies.