The Ukraine Supreme Court today banned the official publication of the presidential election result that officials said handed victory to pro-Moscow Prime Minister Mr Viktor Yanukovich.
The court also said it would examine on November 29th a complaint from the opposition challenger Mr Viktor Yushchenko, alleging ballot rigging.
The ruling has effectively stopped Mr Yanukovich being sworn in as head of state.
The decision could significantly boost Mr Yushchenko supporters who have flooded the streets of Kiev since Sunday's election and have won significant international backing.
European Union foreign policy chief Mr Javier Solana will visit Ukraine tomorrow for urgent talks with the main figures in the country's presidential election crisis, his spokeswoman said tonight.
An Irish flag flies above protesters at an opposition rally in support of Mr Yushchenko in Independence Square in Kiev last night. Photo: Patrick Murray
Mr Yanukovych was declared winner of the election yesterday, but he cannot become president until the results are officially published.
Mr Yushchenko's campaign filed an appeal earlier in the day.
Mr Yushchenko pledged today to press on with mass protests against the election until he has secured victory.
"We will not leave this square until we secure victory," Mr Yushchenko told a crowd of at least 100,000 people a day after election officials declared Prime Minister Viktor Yanukovich president.
"My team and I will stay here even if there are only 15 of us left. We will stand here until our victory."
Earlier, the European Union for an investigation, saying the poll could not be considered democratic according to international standards.Russian President Vladimir Putin, is in The Hague today for a summit with leaders of the European Union. Before the meeting, he congratulated Mr Yanukovich on winning the election, a move certain to strain relations with the West.
"Now the most favourable conditions have been created for the Russia-Ukraine strategic partnership to reach a new and high-quality level," a Kremlin statement quoted Mr Putin's message to Mr Yanukovich as saying.
US Secretary of State, Mr Colin Powell, yesterday challenged Ukrainian leaders "to decide whether they are on the side of democracy or not" and warned of "consequences for our relationship, for Ukraine's hopes for a Euro-Atlantic integration and for individuals responsible for perpetrating fraud."
Tens of thousand of Ukrainians massed in the centre of Kiev for a fourth day todday to support Mr Yushchenko who has called for a "all-Ukrainian political strike" against the declared victory of Moscow-backed Prime Minister Viktor Yanukovich.
The opposition's threat to shut down factories, schools and transportation risk provoking a crackdown by outgoing President Leonid Kuchma, who accused the opposition of trying to carry out "a coup d'etat".
Mr Yushchenko and his followers are protesting against what they see as a fraudulent presidential election, a claim backed by western observers.
Election officials yesterday defied calls by the US and Europe and declared Mr Yanukovich the winner by 49.46 per cent to 46.61 per cent.
With the gulf deepening between the opposition and the government, a key mediator - Mr Lech Walesa, the founder of the Polish Solidarity movement - was expected in the capital to try to help pull the deeply divided nation of 48 million back from the brink of conflict.
A strike could also further divide the country. Mr Yanukovich drew his support from the pro-Russian, heavily industrialised eastern half of Ukraine, while Mr Yushchenko's strength was in the west, a traditional centre of nationalism.
To prevent the crisis from widening, Mr Yanukovich said negotiations with Mr Yushchenko's team would begin today. The opposition has said, however, that it would talk only about a handover of power to Mr Yushchenko, and would only negotiate with Mr Kuchma.
Agencies