Thousands of supporters of Ukraine's Russian-leaning prime minister marched Wednesday to the office of the pro-Western president, protesting a presidential order to hold early elections.

President Viktor Yushchenko on Monday night ordered parliament dissolved, and he called new elections for May 27, plunging Ukraine into its most serious political crisis since the 2004 Orange Revolution.

Prime Minister Viktor Yanukovych has 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," the prime minister said Wednesday.

Yanukovych's supporters have taken over the main Independence Square in downtown Kiev. Elderly people wearing red scarves in support of Yanukovych's coalition partner, the Communists, danced, while younger supporters sat on the edge of fountains and lounged on nearby grass.

"We will be victorious because the law is on our side," said Halina Gusova, 59, a Communist supporter. "And after we win this, our next goal will be to remove the president."

Thousands later marched from the square to Yushchenko's office.

The dispute between Yushchenko and Yanukovych echoed their struggle over the 2004 presidential election, when Yushchenko's supporters erected a tent city in Kiev, claiming the vote was fraudulent.

The protests, which lasted for weeks, became known as the Orange Revolution, after Yushchenko's campaign color. Yushchenko, who was disfigured by dioxin poisoning during the campaign, won a repeat vote. The mystery of how he was poisoned has never been solved.

The Supreme Court played a major role during that electoral dispute. This time, the focus is shifting to the little-known Constitutional Court.

Yanukovych accused the president of trying to influence the court, while Yushchenko's office countered that the prime minister was trying to force the election chief to resign. Both sides denied the charges.

The Constitutional Court said its chief judge had sought to resign, but his resignation was not accepted by the other judges. Yanukovych's supporters said the judge wanted to quit over pressure from Yushchenko's office, an allegation the prime minister denied. The court offered no explanation.

Court spokesman Ivan Avramov said the court has 15 days to render a preliminary decision on whether to accept the case, but the final decision on whether it will be heard must come from a six-judge panel, which faces no time limit.

Yanukovych's supporters have seized the street initiative in the standoff, erecting a tent camp outside the parliament building in addition to taking over the main square. Yushchenko's supporters called off plans for a separate rally on the plaza, saying they wanted to avoid raising tensions.

Yanukovych staged a remarkable comeback last year, when his party won the largest share in parliamentary voting. In August, he put together a coalition that forced Yushchenko to name him prime minister, then expanded it by persuading lawmakers from pro-presidential factions to defect.

Last month, 11 lawmakers joined the ruling coalition, moving it closer to a 300-seat, veto-proof majority in the parliament that would allow Yanukovych's allies to change the constitution.

Yushchenko has called the defections illegal, saying the law permits only blocs, not individual lawmakers, to switch sides. He also says Yanukovych has violated a promise to support the president's policies.

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.