Ukraine's Supreme Court on Thursday prohibited making the results of the nation's disputed presidential election official until it considers an appeal by the opposition, which claims the results were rigged, Ukrainian news agencies reported.

The decision could significantly boost supporters of opposition leader Viktor Yushchenko (search), who have flooded the streets of Kiev since the Sunday run-off and have won significant international backing for their claim the election results giving victory to Prime Minister Viktor Yanukovych (search ) were fraudulent.

Yanukovych was declared winner of the election Wednesday with a margin of about 3 percentage points, but he cannot become president until the results are officially published. The election commission said Yanukovych got 49.46 percent of the vote and Yushchenko 46.61 percent.

Yushchenko's campaign filed an appeal earlier Thursday, but it will not be considered until Monday, the Unian news agency said. That means the ongoing protests and tensions likely will continue for several days.

Court officials could not immediately be reached for comment and it was unclear when the appeal could be decided.

Thousands of Yushchenko supporters have spent four nights outside in the bitter cold to protest authorities' decision to declare Yanukovych the winner. They received a boost Thursday from visiting Lech Walesa (search), the founder of the Polish Solidarity movement, who said he was "amazed" at their enthusiasm and predicted their protest would succeed.

"I hope that Ukraine can avoid the mistakes that Poland made, such as the imposition of martial law," Walesa was quoted as saying by the Polish news agency PAP before leaving Warsaw.

Earlier, leaders of the European Union and Russia, meeting in The Hague , Netherlands, urged their Ukrainian counterparts to find a nonviolent solution to the crisis gripping this former Soviet republic.

Russian President Vladimir Putin (search) — who earlier sent a congratulatory telegram to Yanukovych that his win would raise the two nations' "strategic partnership to a new level" — said after meeting EU leaders in the Netherlands that all claims relating to Ukraine's election should be settled by the courts.

"From my perspective all issues concerning the elections ... should be addressed in accordance with the constitution. All claims should go to the court," Putin said. "We have no moral right to push a big European state to any kind of massive disorder."

Dutch Prime Minister Jan Peter Balkenende (search), whose country holds the EU presidency, said, "We do agree that the peaceful approach to setting up a legitimate government is essential. Any objections to the electoral process must be looked into."

It was unclear, however, whether Yushchenko's appeal to the Supreme Court was legally valid. According to the Interfax news agency, only election results from individual voting districts can be challenged and not results as a whole. The opposition also planned to file complaints in regional courts to protest the vote results.

Policy differences between the two candidates have been overshadowed by the election controversy. Ukraine's economy is considered the fastest-growing in Europe, but looming inflation and rising food prices were a major campaign issue in this nation of 48 million people.

Yushchenko, whose wife is U.S.-born, says he wants to push Ukraine to greater integration with Western Europe, and he has suggested he would seek NATO membership.

His critics worry he will alienate Ukraine from Russia, its key trade partner and main energy supplier.

Yanukovych was praised by Putin and was expected to pursue closer ties with Moscow. Ukraine remains of critical strategic importance to Russia, which is attempting to strengthen its influence over former satellites and considers Ukraine a buffer between Russia and NATO's eastern flank.

However, both candidates support withdrawing Ukraine's troops from Iraq .

Sunday's runoff was denounced as fraudulent by Western observers, who cited voter intimidation, multiple voting and other irregularities. The United States and EU said they could not accept the results as legitimate and warned the Ukrainian government of "consequences" in relations with the West.

Yushchenko sent word to the 15,000 people massed in Kiev's Independence Square on Thursday that the opposition intends to blockade several international highways in western Ukraine, where his support is running high.

Earlier, the reformist candidate called for a general strike to protest the announced result, although businesses and factories in the capital worked as usual Thursday. Some workers reportedly left factories in Yushchenko's stronghold in western Ukraine to come to Kiev.

One hotel in the capital allowed its workers to leave their jobs to join the protests.

The opposition's threat to shut down factories, schools and transportation risked provoking a crackdown by outgoing President Leonid Kuchma (search), who accused the opposition of trying to carry out a coup.

A strike could further divide the country: Yanukovych drew his support from the pro-Russian, heavily industrialized eastern half of Ukraine, while Yushchenko's strength was in the west, a traditional center of nationalism.

To prevent the crisis from widening, Yanukovych said negotiations with Yushchenko's team would begin Thursday. The opposition has said, however, that it would only talk about a handover of power to Yushchenko and would only negotiate with Kuchma.

Thousands of supporters spent a freezing night in the capital, staying in giant tent camps along Kiev's main street and near the presidential administration building. As the sun rose, groups huddled together, drinking hot tea and breaking into regular chants of "Yushchenko! Yushchenko!"

The police presence around the presidential administration building was reinforced Wednesday night as more than 1,000 officers with helmets and shields were bused in.

The building became the site of the most tense standoff yet in the five days of protests when some 15,000 Yushchenko supporters faced off against riot police Tuesday night. Ukraine's Interior Ministry said Thursday it opened a criminal investigation into what it called an attempt to seize the building.