Both sides earlier warned of a civil conflict, as thousands of people continue to protest in Kiev.

The US has said it "cannot accept" Sunday's poll result as legitimate.

Outgoing Ukrainian President Leonid Kuchma - who backs Mr Yanukovych - called on world leaders not to interfere, warning that civil war "could well become a reality at the present time".

In a bid to calm the tense situation, the European Union and Poland have sent envoys to Kiev.

Ukraine's crisis is also set to dominate an EU-Russia summit at The Hague later on Thursday, after the two sides adopted strikingly different stances on the election.

The EU voiced dismay over the result, pointing to widespread reports of election abuses.

Earlier, Russian President Vladimir Putin rushed to congratulate Mr Yanukovych - even before the official result had been announced.

Thousands of opposition supporters braved the cold in a fourth night of protests in the capital, Kiev.

A number of pro-government supporters were also visible in the city for the first time in Wednesday, though eastern Ukraine saw pro-government rallies earlier in the week.

Riot police have been on stand-by since the demonstrations began but there have been no reports of violence.

Ukraine's Central Electoral Commission on Wednesday declared Mr Yanukovych as president, saying he had won 49.46% of the vote against Mr Yushchenko's 46.61%.

However, several exit polls immediately after Sunday's voting suggested that Mr Yushchenko had a clear lead.

Shutting down Ukraine

The pro-Western Mr Yushchenko immediately rejected the commission's declaration as the "latest crime" of the Ukrainian authorities.

He warned a vast crowd of supporters in Kiev's central Independence Square that Ukraine was now on the brink of a "civil conflict".

"With this decision, they want to put us on our knees," he told the crowd, which chanted: "Shame! Shame!"

The nationwide strike aims to shut factories, shops and schools and paralyse Ukraine's major transportation arteries.

Some analysts say the planned stoppage could further divide the country. Mr Yushchenko draws his support mainly from the predominantly Ukrainian-speaking western and central regions, while Mr Yanukovych's strength comes from the Russian-speaking heavily industrialised east.

The opposition is also planning to challenge the official result in the supreme court on Thursday.

Mr Yanukovych - who is backed by Russia - on Wednesday offered to hold talks with Mr Yushchenko.

"We must improve our lives and we will do it together - all of our citizens and myself as president of Ukraine," he said in a brief appearance on state television.

But the opposition camp said Mr Yushchenko would only negotiate with Mr Kuchma.

Washington's warning

In Washington, US Secretary of State Colin Powell said Ukraine was at a "critical moment" and had to decide whether it was on the side of democracy.

Mr Powell warned the government in Kiev of "consequences" for the US-Ukraine relationship.

But he added: "It's still not too late to find a solution which respects the will of the people."

The new head of the European Commission, Jose Manuel Barroso, also warned Ukraine there could be "consequences" for its relations with the European Union, unless there was a serious and independent review.

Western election observers and the Ukrainian opposition have reported thousands of voting irregularities, including a near 100% turnout in some pro-government strongholds.

Earlier, Mr Yushchenko said he was prepared to have a re-run of the vote if it was carried out by "honest" officials.