Ukraine's Prime Minister Viktor Yanukovych has refused to admit defeat in the presidential election won by his rival Viktor Yushchenko.
Mr Yushchenko was quick to claim victory early on Monday
"I will never recognise such a defeat, because the constitution and human rights were violated," he said.
Earlier, international observers praised the conduct of Sunday's re-run vote, held after the second round was annulled over ballot-rigging.
With nearly all votes counted, Mr Yushchenko has an eight-point lead.
The pro-Western opposition leader, who wants Ukraine to push through liberal reforms, is on 52%, against Mr Yanukovych's 44%, official results show.
But Mr Yanukovych said his campaign team had close to 5,000 complaints about how the third round of voting was conducted 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.
The BBC's Steve Rosenberg, in Kiev, says Mr Yanukovich's decision to fight on raises the prospect of fresh legal battles before the final result is confirmed.
Mr Yanukovych, who favours closer ties with Russia, has strong support in Ukraine's industrial east.
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 Organisation 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.
US Secretary of State Colin Powell hailed the "full, free, fair" election and the EU's Dutch presidency said it was "looking forward to a new phase in Ukraine's development", according to AFP news agency.
The Polish President Aleksander Kwasniewski, who mediated in the stand-off, said he was "satisfied to see Ukraine emerge successfully from its political crisis".
Mr Yushchenko declared victory at about 0000 GMT, while the counting was still in its early stages.
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
"This is a victory of the Ukrainian people, the Ukrainian nation," he said. "We were independent for 14 years, today we became free.
"Today, in Ukraine, a new political year has begun. This is the beginning of a new epoch, the beginning of a new great democracy."
Mr Yushchenko then addressed tens of thousands of his supporters in Kiev's central Independence Square, thanking them for their support and urging them to remain in the square until he was officially declared as the winner.
Yushchenko supporters, in their distinctive orange colours, celebrated the anticipated victory with a concert and a fireworks display. They have kept up huge rallies in the capital for weeks, despite sub-zero temperatures.
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 President 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.