Ukraine's Supreme Court has suspended publication of the presidential election result until it considers the opposition's complaints next week.
Yushchenko supporters have kept up their momentum
Opposition leader Viktor Yushchenko appealed to the court after election officials declared Viktor Yanukovych winner of a poll widely seen as rigged.
Mr Yushchenko said he had formed an emergency committee, which observers say could be an alternative government.
Thousands of his supporters also plan to blockade key buildings in Kiev.
Oleksandr Zinchenko, Mr Yushchenko's campaign chief, said a Committee of National Salvation was being set up to "defend the constitution".
He added that Mr Yushchenko had also ordered the creation of a people's self-defence force - the "Ukrainian Guard".
A leading Yushchenko ally, Yuliya Tymoshenko, has urged Kiev demonstrators to block off the parliament, government buildings and the presidential administration.
She said government workers would be on a "forced strike", but said there should be no violence.
'Just the beginning'
Court spokeswoman Liana Shlyaposhnikova told AFP news agency: "Until we have finished examining the appeal, the results of the election cannot be published.
"Until we have reached a decision, the election result cannot be valid."
The court is scheduled to examine the appeal on Monday, the spokeswoman added.
Mr Yushchenko announced the court decision to a rally of thousands on Kiev's main square.
"This is only the beginning - it is proof that it is society that always wins," he declared to wild cheering.
Serhiy Tihipko, an aide to Prime Minister Yanukovych, said that the court did not have the right to cancel the election result and he saw "no justification" for the result to be suspended.
"No-one, not even the Supreme Court, has the right to cancel [the result]," he said.
According to the official election result released on Wednesday, Mr Yanukovych won with 49.46% of the vote against Mr Yushchenko's 46.61%.
Mr Yushchenko's supporters say the authorities oversaw massive fraud in Sunday's election - and independent observers have also reported widespread abuses.
Yanukovych (left): 49.46%
Western observers report:
Abuse of state resources and "overt media bias" in favour of Mr Yanukovych
State workers pressured to give absentee voting certificate to their superiors
Intimidation at some polling stations
Suspiciously high turnout - 96% - in the key pro-government region of Donetsk
Since the strike call, reports speak of major traffic jams on Ukraine's western border where several main roads have been blocked, and some factories and universities are said to have stopped work.
However, coalminers in the east of the country, where Mr Yanukovych's strength lies, have vowed to work on.
Some analysts say the strike could further divide the country as Mr Yushchenko draws his support mainly from western and central regions.
Government supporters including several thousand miners have been holding rallies in Kiev but the capital is still dominated by Mr Yushchenko's supporters.
They have been camping out in Independence Square in freezing temperatures under orange banners.
Ukraine's second national television channel, One Plus One, appears to have dropped the government line and begun broadcasting pictures of the demonstrations in Kiev.
The TV company said it had been forced to broadcast biased information but from now on it would be balanced.
"These principles will be adhered to as long as the channel is on the air," a presenter said before a news bulletin.
Leonid Kuchma, the incumbent president and an ally of Mr Yanukovych, has asked Polish President Aleksander Kwasniewski, with whom he has close ties, and the Lithuanian President Valdas Adamkus to mediate in the crisis.
Lech Walesa, a former Polish president and ally of Mr Yushchenko, has already gone to Kiev to try to help to resolve the crisis, saying he believes a compromise is possible.