Ukrainian President Viktor Yushchenko has dissolved parliament and called a snap election, in an escalation of the country's political crisis.
The move comes amid a power struggle between Ukraine's leaders
The move comes amid a long-running power struggle between the pro-Western president and pro-Russian Prime Minister Viktor Yanukovych.
Lawmakers in parliament said the decree was "a step towards a coup d'etat" and vowed to continue to work.
Mr Yanukovych urged the president to back down on the dissolution.
Analysts say the move could plunge Ukraine into renewed political turmoil.
Thousands of supporters of both sides have been out on the streets in recent days.
The announcement of the new election - set for 27 May - followed seven hours of failed talks between Mr Yushchenko and parliamentary leaders.
Mr Yushchenko accuses Mr Yanukovych of trying to usurp his power by illegally luring pro-Western lawmakers over to his coalition to increase his parliamentary majority.
Under the constitution, only factions - not individuals - can change sides, he says. But last month 11 lawmakers allied with Mr Yushchenko switched sides.
If Mr Yanukovych gains 300 deputies in the 450-seat house, he would have the power to overturn presidential vetoes and oversee new constitutional change.
"My actions are dictated by the strict necessity to save the state's sovereignty and territorial integrity," the president said in his televised address to the nation. "It is not only my right, it is my obligation."
And he accused rivals of using an "unconstitutional process" to form a parliamentary majority. "Deliberate efforts are being made in parliament to worsen the political crisis, posing a threat to our country and people," he said.
Pro-Western opposition leader Yulia Tymoshenko celebrated the dissolution in an overnight rally in Kiev with about 200 supporters. She said the president had made a "courageous decision".
Mr Yanukovych said Mr Yushchenko should not publish the decree, which would mean it would not come into force.
Supporters of Mr Yanukovych have set up camp in the capital
Mr Yanukovych said his rival should instead "sit down at the negotiating table" and that a dissolution would "lead to a significant worsening of the situation in the country".
MPs backed a resolution stating that the legislature would continue to function, and that they would refuse funding for the election.
"The people's deputies have enough courage to withstand blackmail, threats and... ultimatums," parliamentary speaker Oleksandr Moroz said in a statement.
Supporters of Mr Yanukovych have also vowed to defy the president's decision, setting up tents in parks outside parliament.
Over the weekend, tens of thousands of supporters of both factions turned out on the streets of Kiev for rival rallies.
Mr Yushchenko became president in January 2005, following the pro-democracy Orange Revolution which overturned a rigged victory for Mr Yanukovych.
But Mr Yushchenko was forced to accept his rival as prime minister after his allies failed to win a majority in the March 2006 parliamentary election, and the two men have repeatedly clashed.