Ukranian Prime Minister Viktor Yanukovych refused to accept the president's order to hold early parliamentary elections, saying on Wednesday he would wait until the Constitutional Court rules on its legality.
President Viktor Yushchenko ordered parliament dissolved Monday night, and called new elections for May 27.
The president's move created the most serious political crisis since the 2004 Orange Revolution, and plunged the two rival leaders into a sharp political stand-off.
Parliament's majority coalition and the government, led by Yanukovych, have called Yushchenko's decision illegal and appealed to the 18-judge Constitutional Court.
"Until the issue is considered by the Constitutional Court, we will not prepare for elections in any way," Yanukovych said on Wednesday.
Yanukovych accused the president's office of putting pressure on the court's judges, and announced that the court's Chief Justice Ivan Dombrovsky planned to resign. Court spokesman Ivan Avramov said he did not have such information.
The dispute between Yushchenko and Yanukovych echoed their struggle in 2004's bitter presidential race and the subsequent mass protests, when Yushchenko's supporters erected a tent city in the square and remained there for weeks to protest elections which they said fraudulently gave power to Yanukovych.
The protests became known as the Orange Revolution, after Yushchenko's campaign color. During that dispute, Ukraine's Supreme Court played a major role.
This time, the focus is shifting to the little-known Constitutional Court, which has not rendered any decisions in more than eight months.
Avramov said the court has 15 days to render a preliminary decision on whether to accept the case, then a six-judge panel makes a final decision on whether to take the case. That panel faces no time limit.
Meanwhile, Yanukovych's supporters expanded a tent camp outside the parliament building and set tents up on Independence Square.
Yushchenko's supporters had initially planned to hold a separate rally on Independence Square, but Oleksandr Sochkov, spokesman for opposition leader Yulia Tymoshenko, said they called it off to avoid raising any tension.
During Yanukovych's government meeting, Yushchenko's ally in the government, Defence Minister Anatoliy Hrytsenko, countered that it was wrong to wait for the slow-moving court.
He said the presidential order must be fulfilled, and suggested those ministers who disagree should resign.
"We cannot wait for the court's decision which can come in three days or in three months. The president took all responsibility for the order and it must be fulfilled," said Hrytsenko.
Under that pact, reached in August, Yushchenko accepted Yanukovych's return as prime minister in exchange for guarantees he would support the president's domestic and foreign policies.
Yushchenko has since accused Yanukovych of violating that agreement. The United States and Russia have appealed for calm in this nation of 47 million that finds itself caught between its historic ties to Russia and its aspirations to move closer to Europe.
Although Yushchenko and Yanukovych differ over whether Ukraine should join NATO or more closely tie its fate to Russia, much of the wrangling has been widely viewed as efforts by their financial backers and behind-the-scene power-brokers seeking to protect business interests.
Several business clans are known to be vying for influence over lucrative enterprises -- for example, ventures connected to the country's natural gas transport system.
The latest dispute arose after 11 lawmakers joined the ruling coalition last month, moving it closer to a 300-seat, super majority in the Verkhovna Rada that would be veto-proof and could allow Yanukovych's allies to change the constitution.
Yushchenko called the defections "a revision of the voter's will," and illegal, saying the law permits only blocs, not individual lawmakers, to switch sides.
Polls suggest that if parliamentary elections were held today, the leading parties would be Yanukovych's party and the bloc of Yulia Tymoshenko, who was dismissed as Yushchenko's first prime minister in 2005 after a disagreement.
Yushchenko's party would finish a distant third, polls suggest.