According to the ruling, a re-vote for the second round of runoff would be held before Dec. 26, a victory for the opposition camp led by Viktor Yushchenko.

The court made the ruling after five days of hearing of an appeal demanding invalidation of last month's vote, which the opposition said was rigged in the favor of Prime Minister Viktor Yanukovich.

The United States welcomed the decision, saying it is "an important step" to resolve the political crisis in the country.

"The court's decision is an important step in moving toward a peaceful, democratic resolution that reflects the will of the people," White House spokesman Scott McClellan told a news briefing.

"It's important that the will of the people prevail. And the court's decision is part of the political and legal process for meeting that objective," McClellan added.

Both the United States and the European Union, backing Ukraine's opposition, have refused to accept the runoff result as legitimate, and called for an investigation of the "numerous and credible" reports of fraud and abuse in the election.

However, Russia, which supported Ukrainian President Leonid Kuchma and Prime Minister Yanukovich, was frustrated by the court's ruling.

On Friday, Russia's lower house of parliament unanimously approved a resolution condemning what it described as "destructiveinterference" by some European organizations in the political crisis in Ukraine.

The resolution said their "one-sided approach" is provoking radical groups in Ukraine to take dangerous actions "which may lead to massive disturbances, chaos and a split" and will have a negative impact not only on Ukraine, but also on Russia and the world community.

Meanwhile, Moscow also criticized the European Parliament's latest resolution calling for a rerun of the second round of Ukrainian presidential elections, describing it as a manifestationof double standards and interference in Ukrainian internal affairs.

"There is not even a trace of democracy and law in the EuropeanParliament's resolution. It is again about double standards," the Russian Foreign Ministry's information and press department said in a statement Friday.

"The existing Ukrainian law, as is known, does not provide for the possibility of a re-vote for the second round of elections," the statement added.

The ministry also said the European Parliament's call for a re-vote of the second round means nothing but support of one of the opponents.

The European Parliament passed the resolution on Thursday in a vote of 499-67, calling on the Ukrainian authorities "to annul thesecond round of the presidential elections and reorganize this second round before the end of this year."

Ukraine has been locked in a political crisis since the presidential election held in November, in which Yanukovich was declared the winner. But the opposition rejected the result.