Ukraine's PM Viktor Yanukovych has called the president's decision to dissolve parliament a "fatal mistake".
Supporters of Mr Yanukovych have set up camp in the capital
Mr Yanukovych spoke to members of parliament before addressing thousands of his supporters in central Kiev.
Members of parliament are also refusing to obey President Viktor Yushchenko's order to prepare for a snap election.
The pro-Western president accuses his pro-Russia rival of trying to usurp power by increasing his parliamentary majority in an unconstitutional manner.
Mr Yushchenko met Mr Yanukovych in his office, but failed to gain his agreement to implement the decree ordering an early poll on 27 May.
The prime minister's supporters have urged the constitutional court to rule on whether the president's action was legal.
Mr Yushchenko's supporters have been called to demonstrate in the capital, with the stand-off echoing the 2004 Orange Revolution which swept him to power.
The European Commission called for all parties to seek a "peaceful solution" to the dispute.
Presidential poll threat
Speaking to supporters massed outside parliament, Mr Yanukovych threatened to try to force a presidential election if Mr Yushchenko persisted with plans for a snap parliamentary poll.
"He should understand that the only way out is looking for a compromise at the negotiating table," he said.
"There is still a chance to avoid the worst," Mr Yanukovych told parliament, reminding Mr Yushchenko of "the weight of responsibility" on his shoulders.
When the two men met later, the president insisted on a "strict implementation of the decree on an early election", his press service said.
"Viktor Yushchenko, as commander-in-chief of Ukraine's armed forces, also stressed he would allow no use of force in the country," said the president's office.
The announcement of the new election followed seven hours of failed talks on Monday 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. But last month 11 lawmakers allied with Mr Yushchenko switched sides.
If Mr Yanukovych gains 300 deputies in the 450-seat house, he will have the power to overturn presidential vetoes and oversee new constitutional change.
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.