Liberal opposition challenger Mr Viktor Yushchenko said this evening that he did not recognise the election of Ukraine's prime minister as president and called for a country-wide "political strike".
"We do not recognise the election as officially declared," Mr Yushchenko told tens of thousands of supporters massed in Kiev's main square for the third straight day.
Mr Yushchenko said the proclamation of results giving victory to Prime Minister Mr Viktor Yanukovich put Ukraine "on the brink of civil conflict".
Earlier the Ukraine's Central Election Commission declared Mr Yanukovich the winner in a presidential election.
The head of the commission told reporters that Mr Yanukovich has won 49.46 per cent of the vote to 46.61 per cent for liberal opposition challenger Mr Viktor Yushchenko.
The US Secretary of State Mr Colin Powell said today the United States did not accept the results of the disputed election in Ukraine as legitimate and called for immediate action.
Mr Powell urged Ukraine's leaders to "respond immediately" or there would be consequences in the relationship between the two countries.
Outgoing President Mr Leonid Kuchma, meanwhile, warned that civil war could "become a reality" in the country and he asked the world community to stay out of Ukraine's affairs.
Mr Yushchenko said he was prepared to re-run the disputed election provided it was overseen by honest officials.
"We are ready to have a repeat of the second round vote provided we have an honest Central Election Commission," Mr Yushchenko told tens of thousands of supporters massed in Kiev's main square for the third straight day.
The Mr Yushchenko says he was cheated in Sunday's poll.
Up to 200,000 of his supporters protested in the capital Kiev for a third day, marching past buildings housing the presidency, government and parliament and chanting: "Yushchenko! Yushchenko!"
The mass protests engulfed every corner of the city centre and paralysed all normal work. People in apartment buildings opened their windows and waved flags of orange - the campaign colours of Mr Yushchenko - and cheered on supporters. Cars drove by with orange streamers fluttering from radio aerials.
NATO Secretary-General Jaap de Hoop Scheffer today said that Ukraine's disputed presidential election must be reviewed to conform with democratic standards.
"A review of this election is absolutely necessary," the Dutch head of the US-led defence alliance told reporters.
"All NATO asks is a review of this election and sticking to democratic principles, and this is the key to NATO-Ukraine relations," General de Hoop Scheffer said.
The United States and the European Union both urged Ukraine not to certify the election result until claims of fraud are investigated. EU foreign policy chief Mr Javier Solana said Ukraine was at a crossroads over the results and could turn violent.
Late last night, outgoing President Leonid Kuchma called for talks with all parties to solve the election