Ukraine's Parliament met on Saturday in a marathon weekend session to pass legislation needed to rerun a presidential runoff election, a revote ordered by the Supreme Court in a major victory for opposition leader Viktor Yushchenko (search).

The court on Friday threw out the official results of last month's runoff between Yushchenko and his Kremlin-backed rival, Prime Minister Viktor Yanukovych (search), ruling that a revote must be held on Dec. 26. The opposition claimed the runoff was rigged in favor of Yanukovych, and Western governments refused to recognize the results.

Yanukovych's camp said Saturday that the prime minister would participate in the runoff. "There's no other way but to participate and win," Yanukovych's spokeswoman, Anna Herman, said.

Ukraine's Central Election Commission, meanwhile, acted on the court decision and formally set the date for Dec. 26.

Yushchenko's supporters in Parliament were pushing for changes in the election law in hopes of preventing vote fraud and in the membership of the 15-member Central Election Commission.

"We are all hoping for a clean election, and why shouldn't it be? We have shown them what happens when they steal our votes," said 46-year-old Orest Kret, who was decked out in pro-Yushchenko orange near a tent camp where he and tens of thousands of other opposition supporters have massed for 12 days of protests.

The Ukrainian legislature was expected to demand that President Leonid Kuchma fire Yanukovych and appoint a new Cabinet following a non-binding no-confidence vote earlier this week, parliamentary speaker Volodymyr Lytvyn said late Friday.

Friday's court ruling sparked jubilation in the streets of Kiev, with a massive crowd of Yushchenko backers chanting his name, blowing horns and waving balloons and orange flags.

"We have proven that we are a nation that could defend our choice," Yushchenko told his supporters gathered at Independence Square, the epicenter of the opposition protests. "We have changed the nation in the last 12 days. These are different people, they are now citizens."

Yushchenko scheduled another rally for Saturday, urging his supporters not to abandon their demonstration in the square until the government is fired, the election commission replaced and a date formally set for new elections by Parliament.

Thousands of orange-wearing Ukrainians milled around Kiev's tree-lined streets early Saturday, some pausing to tie orange scarves and ribbons around the few remaining Soviet-era monuments not already covered in orange.

The Supreme Court found the runoff was distorted by abuses in pro-Yanukovych regions such as tampering with voters' lists and people voting more than once. It concluded that "the violation of the principles of the election law ... make it unable to determine the voters' will."

The court's decision was a sharp rebuke to Kuchma, who had anointed Yanukovych as his successor, and a slap to Russian President Vladimir Putin, who had strongly supported Yanukovych, fearing Ukraine would tilt further to the West under Yushchenko.

In Yanukovych's stronghold in eastern Ukraine, his backers expressed anger at the court decision. The Donetsk region has already set a January referendum seeking a measure of autonomy, and Yanukovych's representative in court, Stepan Havrysh, warned that Friday's verdict could foment unrest in the east.

In the eastern city of Kharkiv, where regional officials gathered on Saturday to discuss the situation, about 12,000 Yushchenko supporters demonstrated, shouting "We won't be divided!" They outnumbered the quieter gathering of about 8,000 pro-Yanukovych backers, wearing his blue-and-white campaign colors.

Neither Kuchma nor Yanukovych commented on the court's verdict, and Putin, on a state visit to India, also kept mum. A day earlier, Putin had ridiculed Yushchenko's push for a revote.

Boris Gryzlov, the Russian parliamentary speaker who represented Russia in international mediation talks led by the European Union, suggested the Supreme Court decision was no solution.

"The elections in Ukraine have shown that society in that country is split right down the middle, and the Supreme Court's decision in favor of the loser is unlikely to remedy" the situation, Gryzlov was quoted as saying in St. Petersburg by the Interfax news agency.

In Brussels, Javier Solana, the European Union's foreign policy chief, welcomed the court's ruling as "a solution to the political crisis through legal means."

More talks by Yushchenko, Yanukovych and Kuchma with international mediators were expected to be held.