Ukraine's victorious opposition leader Viktor Yushchenko has said it will take time to rebuild relations with Russia, which backed his presidential rival.
Mr Yushchenko has pledged to tackle corruption
Mr Yushchenko, a pro-Western liberal reformer, said Russian-Ukrainian relations had been "distorted" by Ukraine's dominant political clans.
"It's more than a month's work for Ukrainians to restore their old relationship with Russia," he said.
The Kremlin openly backed his opponent, Prime Minister Viktor Yanukovych.
"I really disliked the fact that Russia campaigned in favour of one candidate," Mr Yushchenko told the Russian daily Izvestia.
"This was a serious wound for millions of Ukrainians," he said.
Mr Yanukovych is refusing to admit defeat in the presidential election, which went to a re-run on Sunday, after the second round had been declared invalid by the supreme court due to ballot-rigging.
Election officials on Tuesday confirmed that Mr Yushchenko had won the 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.
But the Russian foreign ministry challenged the objectivity of European election monitors from the Organization for Security and Co-operation in Europe (OSCE).
"Violations characteristic of the first and second round of voting were repeated," said ministry spokesman Alexander Yakovenko.
Despite the tensions, Russian Defence Minister Sergei Ivanov said Mr Yushchenko's victory would not have "any significant impact on our co-operation... in the defence and security field".
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 Mr Yanukovych's threat of court action to dampen their victory celebrations.
Mr Yanukovych, who favours closer ties with Russia, has strong support in Ukraine's industrial east.
Mr Yanukovych recently distanced himself from President Kuchma
In the earlier rounds, he 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.
Official results show that Mr Yushchenko, who wants Ukraine to push through liberal reforms, won with 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, head of the OSCE monitoring mission.
"The people of this great country made a great step forwards to free and fair elections," he told a news conference.