Tens of thousands of opposition demonstrators have flooded back into the heart of the Ukrainian capital Kiev to show support for their leader.
Kiev campers: Yushchenko supporters brave the cold
For a second day people are rallying for pro-western liberal Viktor Yushchenko, saying Sunday's presidential election was rigged.
Official results give Prime Minister Viktor Yanukovych a narrow victory.
Crowds rallied in Kiev's main square and outside parliament, where an emergency debate is due to start soon.
Mr Yushchenko urged more of his supporters to gather outside parliament, despite the freezing cold.
The opposition now want parliament to pass a vote of no-confidence in the central electoral commission and to refuse to recognise the result of the ballot.
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
Mr Yushchenko issued a statement saying he had won a "convincing" victory and calling on nations to recognise him as the winner.
"We appeal to the parliaments and nations of the world to bolster the will of the Ukrainian people, to support their aspiration to return to democracy," he said.
The statement said the opposition would continue "a campaign of civil disobedience" and "a non-violent struggle for recognition of the true results of the election".
The opposition says it has recorded many thousands of irregularities - including very high turnouts in government strongholds.
River of orange
The BBC's Helen Fawkes in Kiev says the road leading to parliament turned into a river of orange - the campaign colour of Mr Yushchenko.
The security forces have warned that they are ready to put down any lawlessness "quickly and firmly".
Ukrainian media reported a crowd of 100,000 protesters in central Kiev.
Another huge pro-Yushchenko demonstration is reported in the western city of Lviv.
In both cities the municipal councils said they would only take orders from Mr Yushchenko, refusing to accept the outcome of the election. They were joined on Tuesday by two more western municipal councils - Khmelnitsky and Lutsk.
The central electoral commission proclaimed Mr Yanukovych the winner, with 49.4% to Mr Yushchenko's 46.7%.
The vote followed days of acrimonious wrangling over the results of the first round.
Several hundred people staged a noisy vigil on the streets of Kiev late into the night despite the bitter cold, shouting "Yushchenko" into loudhailers.
They erected tents and built a fence around them amid rumours that police would try to break up the demonstration in the early hours.
The US State Department said it was "deeply concerned" about the election and threatened to review its relations with Ukraine if the government failed to investigate the allegations of election fraud.
Mr Yanukovych, however, has criticised the call for public protests and said a "small group of radicals" was trying to split the country.
Observers for the Organisation for Security and Cooperation in Europe (OSCE) said Sunday's run-off vote fell far short of European democratic norms.
The organisation, which also reported serious irregularities in the first round, said violations included a continuing "media bias" in favour of Mr Yanukovych and intimidation of observers and voters.
The US' official observer, Senator Richard Lugar, alleged "concerted and forceful" fraud and the EU called on Ukraine to review the election.
However, Moscow, which backed Mr Yanukovych, recognised the result.
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.