Western leaders have welcomed the outcome of Ukraine's presidential election - but Prime Minister Viktor Yanukovych is refusing to admit defeat.
Mr Yushchenko was quick to claim victory early on Monday
Election officials confirmed that pro-Western opposition leader Viktor Yushchenko won Sunday's re-run ballot, eight points ahead of his rival.
But Mr Yanukovych's decision to appeal against the result could revive the legal wrangling over the election.
International observers said the re-run was much fairer than the earlier vote.
Sunday's vote followed a supreme court ruling that the second round result should be cancelled because of widespread ballot-rigging.
But Mr Yanukovych, who emerged on top in the second round, said constitutional and human rights were violated in the re-run.
"I'm certain that the winner of this election was not Viktor Yushchenko," he told the BBC.
Mr Yanukovych, who favours closer ties with Russia, has strong support in Ukraine's industrial east.
But the BBC's Steve Rosenberg in Kiev says the crowds of Yushchenko supporters on the streets of the capital, rallying with orange scarves and banners, are not allowing the threat of court action to dampen their victory celebrations.
Official results show that Mr Yushchenko, who wants Ukraine to push through liberal reforms, is on 52%, against Mr Yanukovych's 44%.
But Mr Yanukovych said his campaign team had close to 5,000 complaints about the conduct of the re-run and would appeal to the supreme court.
He pointed to the reported deaths of eight voters, linking them to a last-minute constitutional court decision on voting rights for the disabled.
US Secretary of State Colin Powell hailed the election as "full, free, fair" and the EU's Dutch presidency said it was "looking forward to a new phase in Ukraine's development".
THIRD TIME AROUND
Repeat of 21 November run-off after cancellation of result
Fourth presidential election since independence in 1991
36 million voters; 12,000 foreign election observers
The Polish President, Aleksander Kwasniewski, who had mediated in the election stand-off, said he was "satisfied to see Ukraine emerge successfully from its political crisis".
Nato Secretary General Jaap de Hoop Scheffer said the election was "relevant to Nato's political relationship with Ukraine" and its aim to promote regional stability.
Sunday's election re-run was monitored by 12,000 international observers.
"The Ukrainian elections have moved substantially closer to meet OSCE standards," said Bruce George, the head of an Organization for Security and Co-operation in Europe (OSCE) monitoring mission.
"The people of this great country made a great step forwards to free and fair elections by electing the next president of Ukraine," he told a news conference.
Mr Yushchenko declared victory early on Monday, while the counting was still in its early stages.
Many Ukrainians, going back to the polls for the third time in less than two months, said they wanted to end the country's bitter political crisis.
In the earlier rounds, Mr Yanukovych was backed by outgoing President Leonid Kuchma and Russian President Vladimir Putin.
But last week Mr Putin said he would accept, and work with, whoever won.
And Mr Kuchma suggested the loser should concede within two days. "Dear God, let this be the final vote. I'm sure it will be," he said.