Kiev, Ukraine ? Ukraine’s disputed presidential runoff election was ruled invalid Friday by the nation’s Supreme Court and a revote was ordered by Dec. 26, a decision that sent thousands of jubilant backers of pro-Western opposition leader Victor Yushchenko into the streets in celebration.

The court declared that the Nov. 21 runoff, in which pro-Russian Prime Minister Viktor Yanukovych led Yushchenko by a slim margin, had been marred by polling place fraud.

Parliament was scheduled to meet today and Sunday to consider legislation needed to implement the court ruling. It was expected to pass legislation aimed at preventing fraud in the repeat runoff, in particular a ban on absentee ballots.

Yushchenko supporters, convinced that his path to the presidency was now clear, gathered with him in Kiev’s main square for a mass rally.

“The judges of the Supreme Court are the true heroes today,” said Yushchenko, his two young daughters on the stage next to him, one of them hugging his leg. “Justice, the rule of law and freedom have started to return to Ukraine, thanks to today’s decision.”

There was no immediate reaction from Yanukovych, who had been officially declared the winner of the runoff that opposition and foreign observers said was rigged.

But Stepan Gavrysh, one of the prime minister’s representatives on the Central Election Commission, complained to reporters that, by demanding a revote, the court exceeded the bounds of the case it was hearing. The opposition appeal had asked the court to annul the presidential runoff and declare Yushchenko winner based on his narrow victory in the multi-candidate first round held on Oct. 31.

“This is a political decision, a way to resolve the political crisis, but I am not sure that it will be successful,” Gavrysh said. “The court exceeded the limits of the complaint from Viktor Yushchenko, who did not ask for another ballot.”

The Russian news agency Interfax reported that when asked whether Yanukovych would agree to compete in a revote, Gavrysh replied, “I don’t know.”

Supporters of Ukrainian opposition leader Viktor Yushchenko celebrate in the main square of Kiev. The Supreme Court declared the results of Ukraine's disputed presidential run-off election invalid Friday and ruled that the run-off should be repeated Dec. 26.

Among the possible scenarios for the pending vote were “an easy victory for Viktor Yushchenko” and “a difficult victory for Viktor Yanukovych,” Gavrysh added.

In Washington, the State Department welcomed the ruling.

“We certainly agree with the decision … that the last round was marked by significant fraud and that it therefore can’t be upheld as a fair result,” spokesman Richard Boucher said. “What is important now is to move ahead quickly … to ensure a new vote that is fair, free and that results in an outcome that reflects the will of the Ukrainian people.”

Outgoing President Leonid D. Kuchma in recent days endorsed the idea of more voting as the way out of the crisis, but said it must be an entire new two-round election open to new candidates.

Yushchenko told the Independence Square rally that the court ruling “put a full stop” to that idea. The crowd responded by chanting, “Away with Kuchma!”

Kuchma made no immediate comment, but in recent days both sides have said they would abide by any Supreme Court ruling. An agreement among Kuchma, Yanukovych and Yushchenko, reached with the help of European mediators, linked some form of new election with political reforms that would shift some power from the president to the prime minister. It was not immediately clear how strictly action on those reforms, which need to be enacted by parliament, might be linked to meeting the Dec. 26 deadline for a revote.

Meanwhile parliament, exercising increasing independence from Kuchma, voted 257-0 to pull the country’s 1,600 troops out of Iraq. While the vote is nonbinding, both presidential contenders had made campaign pledges to pull out the troops.

Yushchenko is widely seen as a pro-Western democratic reformer, while Yanukovych, whose power base is in the country’s largely Russian-speaking east, has received Moscow’s backing. But the presence of Ukrainian soldiers in Iraq is highly unpopular here. Parliament cited a “sharp deterioration of the situation in Iraq” in justifying its pullout resolution.

Despite likely displeasure in Washington over any Iraq pullout, a Yushchenko victory would be expected to move this nation of 48 million toward warmer ties with Western Europe and the United States. Western governments have provided critical backing for his charge that the runoff tally was fraudulent and that a revote must be held.

Within Ukraine, Yushchenko has the fervent support of those who want the country to follow the path of post-communist European integration blazed by former Soviet satellite states like Poland, Hungary and the Czech Republic, all now members of the European Union and NATO.

By law, balloting must be held on a Sunday, and the Supreme Court ruling said it must come within three weeks of the coming Sunday. That was widely interpreted to mean that the revote would be held on the last possible day, Dec. 26, since time is needed to prepare for the balloting.

If Yanukovych withdraws from the runoff at least 10 days before the balloting, he would be replaced by the first round’s third-place finisher, Socialist Party leader Oleksandr Moroz, who joined Yushchenko’s side after the first round. If Yanukovich were to withdraw less than 10 days before the balloting, Yushchenko would run unopposed, and would need to win a majority of votes cast to become president.

Yushchenko urged Kuchma to act quickly to fire Yanukovich and his Cabinet in line with a parliamentary no-confidence vote Wednesday. He charged that the Central Election Commission had “betrayed” the nation by endorsing a fraudulent election, and demanded that its makeup be revised for the new balloting.

Yushchenko also blasted Kuchma for making a quick trip to Moscow Thursday, where he won Russian President Vladimir V. Putin’s endorsement for an entire new election, open to new candidates, rather than a repeat of the runoff round as favored by the opposition.