Ukraine's Parliament prepared for a marathon weekend session to pass legislation needed to hold a rerun of 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 revotwas rigged in favor of Yanukovych, and Western nations refused to recognize the results.

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

The legislature was also expected to demand that President Leonid Kuchma fire Yanukovych and appoint a new Cabinet following a 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 — his campaign color.

"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 the election commission and Parliament.

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.

Yanukovych's representative in court, Stepan Havrysh, said the verdict was a "political decision" that wasn't backed by evidence.

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 rerun.

Boris Gryzlov, the Russian parliamentary speaker who had represented Russia in international mediation talks led by the European Union, suggested the Supreme Court decision was not a 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 EU'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 in Kiev on Saturday.

It was unclear whether Solana would come for the third round of talks, but the presidents of Poland and Lithuania — Alexander Kwasniewski and Valdas Adamkus — and Jan Kubis, the chief of the Organization for the Security and Cooperation in Europe, were expected in Kiev later in the day.

Ivan Kuras, Yanukovych's ally in Parliament, said that the prime minister could withdraw from the race, the Interfax news agency reported. Havrysh said he doesn't know whether Yanukovych will run but added that he could still win if he does.

According to Ukrainian law, if one of two rivals in the runoff steps down more than 10 days before the vote, the first-round contender who came in third is put on the ballot. If Yanukovych left the race, socialist leader Oleksandr Moroz, who had thrown his support behind Yushchenko in the Nov. 21 runoff, would take his place.

If Yanukovych withdraws from the race less than 10 days before the vote, the law would require Yushchenko to get more than 50 percent of ballots to win the presidency.

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