KIEV, Ukraine — The Supreme Court declared the results of Ukraine's disputed presidential run-off election invalid Friday and ordered a new run-off be held on Dec. 26, sparking a burst of cheers and fireworks from tens of thousands of opposition supporters rejoicing in Kiev's main square.

The ruling, made after five days of hearings by the court's 21 justices, was a major victory for opposition leader Viktor Yushchenko, who had rejected the government's demands that an entirely new election be held.

And it was a stinging blow to outgoing President Leonid Kuchma and his powerful ally, Russian President Vladimir Putin, who wants to preserve Moscow's centuries-old influence in Ukraine in the face of Yushchenko's followers' desire to move closer to the West. Only a day earlier, Putin had sharply derided the idea of holding a new run-off.

The opposition had pinned its hopes on the court's ruling in its bid to overturn the results of the Nov. 21 run-off vote, in which Kuchma ally Prime Minister Viktor Yanukovych was declared the winner. The opposition said the vote was rigged to cheat Yushchenko of victory.

"Today Ukraine has turned to justice, democracy and freedom," Yushchenko told the throng of supporters who have packed the capital's Independence Square for 12 days. "It happened thanks to you."

"The Supreme Court has put a big final stop to the new election that Kuchma wanted," said Yushchenko, wearing an orange scarf, his campaign color. He urged Kuchma to fire Yanukovych and his Cabinet and demanded a reshuffle of the Central Election Commission, which he said "betrayed" the nation by endorsing the fraudulent vote.

Yushchenko waved his clasped hands over his head like a victorious prizefighter, and the crowd, fluttering orange banners and blue-and-yellow Ukrainian flags, burst out with the national anthem, with Yushchenko joining in putting his hand over his heart as he sang along.

Kuchma had been pressing for an entirely new election instead of a new run-off, apparently in hopes of replacing Yanukovych with a stronger candidate to run against the opposition leader.