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, ruling that a revote must be held on Dec. 26. The opposition had claimed the runoff was rigged in favor of Yanukovych, and Western nations refused to recognise 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.