The supreme court rejected official publication of results that showed Prime Minister Viktor Yanukovich had beaten Viktor Yushchenko in a run-off election on Sunday.
The ruling effectively stopped Mr Yanukovich being sworn in as head of state. The West-leaning Mr Yushchenko, who says he was robbed of victory by electoral fraud, hailed the ruling as a victory.
“This is only the beginning. It is proof that it is society that always wins. It is small compensation for the suffering that we have endured,” he told tens of thousands of cheering supporters on Kiev’s Independence Square.
Mr Yushchenko also received a boost yesterday from visiting Lech Walesa, the founder of the Polish Solidarity movement, who said he was “amazed” at their enthusiasm and predicted their protest would succeed.
Former Polish president Mr Walesa, who arrived in Kiev to try to help pull the deeply divided nation of 48 million back from the brink of conflict, said held talks with Mr Yanukovych, “not as a presidential candidate but as prime minister. If he is prime minister, then he should unify and not divide the people”.
Mr Yanukovich repeated an offer for talks with his rival. The election, he said, had been fair, and western countries had no right to condone opposition calls for further demonstrations
The four days of high tension since the election have led to warnings of civil conflict by Western officials and even the country’s outgoing president, and spawned rumours of a violent backlash from Mr Yanukovich’s supporters.
The court, which has shown it is independent-minded in the past, also said next Monday it would examine Mr Yushchenko’s complaint that the election had been rigged.
Under the court ruling, outgoing President Leonid Kuchma, who has been in power for 10 years and had endorsed Mr Yanukovich, would stay on in power for now.
Mr Yushchenko has said he is ready to take part in fresh elections so long as rules are tightened.
Encouraged by the supreme court ruling, Mr Yushchenko’s political allies announced they would ratchet up the pressure.
Firebrand deputy Yulia Tymoshenko urged supporters to begin a peaceful blockade of the government building and parliament.
In The Hague, Holland, yesterday, Ukraine’s crisis dominated summit talks between Russian President Vladimir Putin and the EU presidency, which are at odds on the issue.
The West has made clear to Ukraine it regards the election as fraudulent.