Ukraine's Supreme Court yesterday blocked the inauguration as president of the Prime Minister, Mr Viktor Yanukovich, agreeing to opposition calls for an investigation into claims of massive fraud in last Sunday's elections.
The news was greeted with roars of delight by a crowd of 100,000 protesters packed into Kiev's Independence Square who insist their leader, Mr Viktor Yushchenko, is the rightful president.
The European Union foreign policy chief, Mr Javier Solana, arrives in Ukraine today for talks with Mr Yanukovich, Mr Yushchenko and the outgoing President, Mr Leonid Kuchma, to discuss "a negotiated diplomatic solution" to the dispute.
The Supreme Court ruling was backed by the EU and Russia, papering over their previous split on the issue at their summit meeting in The Hague. Mr Vladimir Putin, who earlier this week congratulated Mr Yanukovich on his election victory, said he supported the Supreme Court's decision. But he also said outsiders should not push Ukraine into "mayhem".
"We don't believe it is our right to interfere in any way in the electoral process or impose our opinion on the Ukrainian people," he said.
This decision defuses, at least for the coming days, the growing tension in Ukraine between government and opposition caused by the disputed elections.
Hopes are high among opposition supporters that the judges will now examine the volumes of evidence of fraud compiled by international monitors.
But Mr Yushchenko's political allies showed they are determined to keep up the pressure. His deputy, Ms Yulia Tymoshenko, urged supporters to begin a peaceful blockade of the government building and parliament. As a result, people were allowed to leave the buildings last night but not to re-enter them.
Many in the crowd, braving temperatures of minus 10 degrees Celsius, said they were worried the government would exert political pressure on the judges, and vowed to continue their protests.
"This is a victory for the opposition. Maybe not the whole way, but it's a step ahead," said Nina (45), a Kiev shopkeeper. "We have not yet won. We will stay here until we know for sure our president has won." A president cannot be sworn in without the result being officially published.
"The court ruling bars the Central Election Commission from officially publishing the results of the election and proceeding with any other action connected with this," the court said in a statement.
Mr Yushchenko vowed that protests would continue throughout the weekend, when huge demonstrations are planned. "We will not leave this square until we secure victory," he told cheering protesters.
His call for a general strike fell mostly on deaf ears. Some businesses and government offices closed, but the opposition urged transport workers to remain working to ensure that tens of thousands of opposition supporters could be ferried around the country. And in the east, which is Ukraine's industrial powerhouse, miners loyal to the government refused to down tools, pledging support for Mr Yanukovich.
By yesterday evening supporters of Mr Yushchenko, wearing orange ribbons and scarves, could be seen in animated conversation with smaller bands of government supporters with blue hats and flags.
The news eclipsed the arrival of Mr Lech Walesa, the former Polish president, who announced he had arrived to mediate in the dispute. He appeared in front of crowds on a podium in Independence Square to cheer them.
The current Polish President, Mr Aleksander Kwasniewski, announced that he would head for Ukraine within days to help mediate. Such mediation will probably remain on ice pending a ruling by the Supreme Court, which is expected to start considering the case on Monday, with a decision possible next Friday.
The Dutch Prime Minister, Mr Jan-Peter Balkenende, whose country holds the EU Presidency, insisted that alleged electoral fraud must be investigated.