IEV, Ukraine, Dec. 3 - Ukraine's Supreme Court overturned the results of the country's disputed presidential election on Friday, saying they were marred by "systemic and massive violations," and ordered a new runoff between the main candidates by Dec. 26.

The court's ruling - announced just before 6 p.m. after a day of suspense that halted Parliament's deliberations as well as talks aimed at a resolution of the impasse - was a surprising and decisive victory for Viktor A. Yushchenko, the opposition leader who asserted that he had been denied his rightful victory.

It came 12 days after tens of thousands of Ukrainians poured into the streets of Kiev and other cities and began round-the-clock protests denouncing what they considered a violation of their right to choose a successor to Leonid D. Kuchma, the departing president.

The streets, which had remained electric despite the tumult of seesawing momentum, exploded in jubilation. Thousands more poured into Independence Square in Kiev, forming a crowd larger than any other since the Nov. 21 runoff election that Prime Minister Viktor F. Yanukovich was declared to have won.

Beneath exploding fireworks, they celebrated a democratic triumph in a country of 48 million that only 13 years ago emerged from the Soviet Union with no democratic experience or traditions.

"Today we may say that Ukrainians - not only those who are here, but also those who see us on television and listen to the radio - began turning to justice, law and freedom," Mr. Yushchenko said. Speaking as if he had already won the presidency, though he must face the voters again, he called on Mr. Kuchma and Mr. Yanukovich to resign.

The court's decision signaled a turning point in a political and diplomatic crisis that paralyzed the government, rattled the economy and raised the specter of a civil conflict between the country's starkly divided regions, dominated by ethnic Ukrainians in the west and ethnic Russians in the east. The election also exposed tensions between Russia and the West not seen since NATO bombed Serbia in 1999, and perhaps since the cold war.

The ruling abruptly ended Mr. Kuchma's efforts, endorsed by President Vladimir V. Putin of Russia on Thursday, to hold a new election from scratch, having implicitly conceded that the previous one would never produce a legitimate victory for Mr. Yanukovich. A completely new election would allow a new field of candidates and possibly a new pro-government, pro-Moscow candidate replacing Mr. Yanukovich.

Mr. Kuchma and Mr. Putin, whose support for Mr. Yanukovich has provoked angry protests here and abroad, mocked the idea of another runoff as impractical, in a meeting outside Moscow on Thursday night. Several European leaders, by contrast, called for a new runoff, saying they were not supporting Mr. Yushchenko as much as a free and fair electoral process.

Mr. Yushchenko, on Independence Square, derided Mr. Kuchma's visit to Russia. "On the most crucial day for Ukraine, Leonid Kuchma went for advice not to his own people, but abroad," he said, prompting chants of "Kuchma out!"

The ruling appeared to stun the two men who, for now, make up the core of Ukrainian state power. Mr. Kuchma, who has served as president for 10 years, surviving protests and scandals that tarnished much of his legacy, did not immediately address the court's decision. His spokeswoman, reached by telephone, declined to say anything.

Mr. Yanukovich, the prime minister since 2002, also did not respond, though a spokeswoman, Hanna German, remained defiant and confident. She said he would run again - and prevail. But as the crisis unfolded, Mr. Yanukovich appeared to lose the backing of Mr. Kuchma and other important supporters, including his campaign manager, Sergey Tyhypko.

"He is sure he will win anyway," Ms. German said. "They can run the election five times, 18 times and anyway he will be the winner."

The Supreme Court, which has 85 judges in four chambers and a military collegium, has in the past displayed some independence from Mr. Kuchma's centralized power. Parliament elects judges to life terms, a basic step toward independence. Like judiciaries across the former Soviet Union, though, the court has been considered susceptible to government influence and pressure.

Only six days ago, a senior Western diplomat discounted the possibility that it would rule objectively on Mr. Yushchenko's appeal.

"Historically the Supreme Court is subject to bribery and intimidation," the diplomat said, speaking on condition of anonymity because of diplomatic protocol and the sensitivities surrounding the crisis. "It is as simple as that. No one has ever considered it an institution subject to independence and integrity."

Twenty-one of the court's judges began hearings on Monday and continued through the week in full view of the nation. The proceedings were viewed on television in virtually every public place in Kiev, even on one channel, One Plus One, that before the election had carefully hewed to the government's positions.

Lawyers for Mr. Yushchenko presented what they called voluminous evidence of violations of campaign and election laws, often in numbing detail. On Wednesday, Mr. Yanukovich unexpectedly filed his own appeal, accusing Mr. Yushchenko's campaign of election violations. While his appeal appeared to be a tactic to counter accusations of ballot stuffing on Mr. Yanukovich's behalf, it bolstered arguments that the election, over all, had not been fair.

The judges accepted most of Mr. Yushchenko's accusations that the government's manipulations before and during the voting had prevented a free and fair election, something called for in the Constitution. Mr. Yushchenko's lawyers based most of their case on constitutional issues, not the specifics of the election law.

The judges began deliberating on Friday morning after hearing final arguments and, after voting in chambers, announced the ruling. The judges' vote was not disclosed.

Roman M. Zvarich, a member of Parliament and a lawyer who represented Mr. Yushchenko in the court's crowded chambers, said the judges had displayed a democratic maturity by establishing for the first time a larger precedent based on the primacy of constitutional rights above the election or other laws.

"The court took the initiative to fill in gaps in the election laws," he said. "This is a milestone decision. The court took a very courageous stand."

Mr. Yanukovich's allies criticized the ruling as blatant overreaching. Valentin Konovaluk, a member of Parliament, said the court had exceeded its authority and even Mr. Yushchenko's challenge, which called for invalidating the results but not calling for a new election.

"It is another political decision that only confirms that we have all crossed the boundaries of existing legislation and law," he said.

The court ruled that under current law, the new runoff should be held within three weeks of Sunday, but appeared to leave the exact date to Parliament, which is scheduled to be in emergency session again on Saturday to take up the issue. Mr. Yushchenko's supporters have pushed for a vote as soon as Dec. 12 or 19, but most officials assumed that it would be held on Dec. 26, given the time needed to organize the balloting.

The court left other political issues unresolved.

The ruling, read by Anatoly Yarema, the presiding judge, cited "unlawful" misconduct by the central election commission that made it "impossible to declare who won." Mr. Yushchenko's supporters quickly seized on that to demand criminal prosecutions of the commission's members. They also reiterated their demands that Mr. Kuchma dismiss the entire commission and appoint a new one, as well as new ones in the regions, before a new election was held.

The court's ruling reverberated beyond the nation's borders. The United States and European countries, having denounced the election results, welcomed the decision.

Mr. Putin, traveling in India, did not immediately react. In Moscow, Russia's Parliament adopted a resolution before the court's ruling that accused the European Union and the Organization for Security and Cooperation in Europe of fomenting unrest in Ukraine. The speaker of Parliament, Boris V. Gryzlov, represented Russia at the negotiations overseen by those institutions.

Grigory A. Yavlinsky, the leader of the liberal party Yabloko, reflected the divisions in Russia prompted by Ukraine's electoral crisis. At least a few on the margins of official power in Russia have found hope in the events unfolding in their Slavic neighbor.

"For the first time in the territory of the former U.S.S.R., a top judicial body has rejected falsification, the use of administrative resources and mockery of the people's will," he told the Interfax news agency.