Tens of thousands of Ukrainians have thronged major cities, protesting at an election result they consider flawed.
Opposition supporters filled the centre of Kiev
Opposition leader Viktor Yushchenko told supporters to stage a civil disobedience campaign.
The cities of Kiev and Lviv refused to recognise the official victory for Prime Minister Viktor Yanukovych.
While Russia accepted the result, a US observer alleged "concerted and forceful" fraud, and the EU called on Ukraine to review Sunday's election.
"We are very concerned about the news we have had about the outcome," said Dutch Foreign Minister Bernard Bot, representing the EU.
He said all 25 EU member states would be calling in
Ukrainian ambassadors "to convey our message of serious concern".
Moscow, which backed Prime Minister Yanukovych, recognised the election result.
"I have congratulated Viktor Yanukovych for his victory," said President Vladimir
Putin's personal envoy, Boris Gryzlov.
Observers for the Organisation for Security and Cooperation in Europe (OSCE) said Sunday's run-off vote fell far short of European democratic norms.
Mr Yushchenko, seen as the pro-Western candidate, told thousands of people who braved sub-zero temperatures in the capital, Kiev, not to leave the rally "until victory".
"We are launching an organised movement of civil resistance," he said, denouncing what he called the "total falsification" of the vote which followed days of acrimonious wrangling over the results of the first round.
Kiev city council refused to recognise the results, and urged parliament to follow suit.
Thousands of people turned onto the streets in the
western city of Lviv, where the city council said it would only take orders from Mr Yushchenko.
Imprisoned twice in his youth
Former governor of industrial Donetsk region
Raised pensions and public sector pay before election
Would make Russian second official language and allow dual citizenship
An economist and former central banker
Has an American wife
Promises to fight corruption, create five million jobs and pursue free market reforms
Would seek deeper relations with Europe and the West
The central electoral commission said with more than 99% of the vote counted, Mr Yanukovych had 49.4% while Mr Yushchenko had 46.7%.
But the opposition says it has recorded many thousands of irregularities - including very high turnouts in government strongholds.
Mr Yanukovych was backed by incumbent President Leonid Kuchma.
Exit polls earlier suggested that Mr Yushchenko had been on course for victory with a lead of at least six percentage points.
"The second round did not meet a considerable number of [international] commitments for democratic elections," said Bruce George, head of the OSCE mission in Kiev.
The OSCE also reported serious irregularities in the first round.
"The abuse of state resources in favour of the prime minister continued, as well as an overwhelming media bias in his favour," Mr George said.
The group said Sunday's violations also included intimidation of observers and voters.
The authorities are investigating the killing of a policeman who was guarding ballot papers in a village in central Ukraine. The motive for the killing is not known.
Mr Yushchenko's supporters say they do not believe the official turnout figure of 96% in eastern Ukraine.
"I believe in my victory but the government... has staged total fraud in the elections in the [eastern] Donetsk and Lugansk regions," Mr Yushchenko said.
Kiev was on high alert, with extra police and soldiers on the streets, and riot vehicles outside the central electoral commission.
During the campaign, Mr Yushchenko, prime minister between 1999 and 2001, claimed to have been the victim of intimidation and dirty tricks, including an alleged poisoning attempt.
His critics portray him as an American puppet who will do anything to gain power, including inciting civil unrest.