Thousands of demonstrators have taken to the streets of Kiev to protest against the Ukrainian president's decision to dissolve parliament and call for elections.
Viktor Yushchenko, the president, on Tuesday stood by his orders and appealed for support from diplomats, law enforcers and governors as his standoff with the prime minister escalated.
At a four-hour meeting in Yushchenko's office, Viktor Yanukovich, the prime minister, was told that "according to the constitution, a presidential order must be fulfilled".
But he rejected the dissolution order and his backers marshalled thousands of supporters to gather in the capital.
Yushchenko called for elections to be held on May 27 but Yanukovich urged him to return to negotiations.
Legality
The prime minister's party also plans to challenge the legality of the dissolution order.
On Tuesday, Yushchenko met police and military leaders, telling them that their "key mission is to stay out of politics".
The political crisis is the most serious since Yushchenko swept to power in 2004 on the back of the Orange Revolution which rejected Russian interference in Ukrainian politics.
But since the victory Yushchenko has seen his power eroded by Yanukovich, the man he defeated in 2004.
A breakdown of a power-sharing agreement on Monday between Yushchenko, who has the West's support and Yanukovich, supported by Russia, led to the standoff.
Rivalry
Yushchenko accused the prime minister of failing to offer guaranteed support for his domestic and foreign policies, as agreed in August, in exchange for returning to office.
The rivalry between the two has remained sharp throughout the eight months they have shared power.
Although the leaders have different opinions over whether Ukraine should join Nato, much of the wrangling is seen as efforts by their financial backers seeking to protect business interests.
Yushchenko's supporters who held a big demonstration over the weekend said late on Tuesday that there were no plans to organise new rallies.
The leaders are seeking help from Europe to resolve the standoff.