Ukraine's election commission has declared Prime Minister Viktor Yanukovych the official winner of the disputed presidential election.
The opposition supporters show no signs of backing down
Supporters of opposition leader Viktor Yushchenko, who say the vote was rigged, have surrounded the commission building for a third day of protests.
Correspondents say there are fears the peaceful rallies could become violent.
US Secretary of State Colin Powell said Washington "cannot accept" the result as legitimate.
"It's still not too late to find a solution which respects the will of the people," Mr Powell told reporters following the announcement in Ukraine.
He said Ukraine was at a "critical moment" and had to decide whether it was on the side of democracy.
Viktor Yanukovych (left): 49.46%
Viktor Yushchenko: 46.61%
Without action, there might be "consequences" for the US-Ukraine relationship, he added.
Mr Yushchenko has claimed massive electoral fraud and declared himself the rightful winner.
"I speak to you as the president of Ukraine," he told supporters gathered in freezing temperatures in Kiev's Independence Square.
A number of pro-government supporters are also on the streets and the two sides have been trading taunts. Riot police are on stand-by.
Following the official election result, a number of people started to leave the main square and head in the direction of the presidential offices.
The election commission said Mr Yanukovych won Sunday's second round vote with a margin of almost three percentage points.
The commission had already indicated a win for Mr Yanukovych, but exit poll results had put Mr Yushchenko ahead.
Opposition MPs at Wednesday's meeting chanted "shame" and "lies" after the announcement.
The US and the European Commission had urged Ukraine not to announce the result before reviewing the contentious vote.
The new head of the European Commission, Jose Manuel Barroso, earlier warned Ukraine there could be "consequences" for its relations with the European Union, unless there was a serious and independent review.
The Netherlands, which holds the rotating presidency of the EU, has announced it plans to send an envoy to Ukraine to discuss the dispute result.
Polish President Aleksander Kwasniewski has also sent a top foreign policy adviser.
'Need credible results'
Mr Yanukovych had not declared himself the winner before the official confirmation and has said very little since the vote.
Earlier, he broke his public silence to say he did not want a "fake victory".
"I will not accept the results of the presidential election until it is proved to me and the Ukrainian people that they are legitimate and credible in accordance with conditions set down by the constitution," he said in a statement.
"I need no fictitious victory, a result which could lead to violence and victims. No position of authority, no matter how important, is worth a single human life."
The opposition says it has recorded thousands of voting irregularities in the poll, including a near 100% turnout in some pro-government strongholds.
Western election observers have also reported mass violations.
Talk of a re-run
Earlier, Mr Yushchenko told his supporters he was prepared to have a re-run of the vote if it was run by "honest" officials.
He said the opposition would also demand that a law be signed banning the use of absentee ballots, which the opposition says were used to allow pro-Yanukovych supporters to vote multiple times.
A BBC correspondent says that given the fraud allegations, the opposition seems unlikely to agree in practice to a new election if it was run along the same lines as before.
Mr Yanukovych told his cabinet on Wednesday he does not believe anything extraordinary is taking place in the country.
"It is necessary that people live in the normal mode. Therefore, we must carry our constitutional duty and ensure the vital activities of state," he said.