World leaders have voiced their support for Ivory Coast opposition candidate Alassane Ouattara, saying he is the true winner of a presidential run-off.
The country's Constitutional Council has overturned results to declare President Laurent Gbagbo the winner - he is due to be sworn in shortly.
The US, UN, France and the West African bloc Ecowas have urged Mr Gbagbo to accept defeat.
The poll was intended to reunify the country after a civil war in 2002.
Prime Minister Guillaume Soro warned the overturning of the results threatened to derail attempts to stabilise and re-unify the country.
On Thursday, the independent Ivorian Election Commission declared that Mr Ouattara had won the 28 November run-off by 54.1% to 45.9%.
But after Mr Gbagbo and his supporters alleged the ballot had been fraudulent, the Constitutional Council overruled the Commission.
Chairman Paul Yao N'Dre said Mr Gbagbo had secured just over 51% of the vote.
Ivorian state media said Mr Gbagbo would be sworn in at a midday ceremony on Saturday.
'Held to account'
US President Barack Obama has rejected the Council's decision.
"Independent Electoral Commission, credible and accredited observers and the United Nations have all confirmed this result and attested to its credibility," he said.
He congratulated Mr Ouattara and said the international community would "hold those who act to thwart the democratic process and the will of the electorate accountable for their actions".
President Nicolas Sarkozy of France - the former colonial power in Ivory Coast - told Mr Gbagbo to "respect the will of the people, abstain from any action that might provoke violence" and to help establish peace.
UN Secretary General Ban Ki-moon earlier called on Mr Gbagbo "to do his part for the good of the country and to cooperate in a smooth political transition".
The chairman of regional bloc Ecowas, Nigerian President Goodluck Jonathan, said all parties should "respect and fully implement the verdict of the Ivorian people as declared by the Independent Electoral Commission".
The head of the UN mission in Ivory Coast also said it regarded Mr Ouattara as the winner, while the African Union said it was "deeply concerned" by the developments.
Mr Ouattara told reporters on Thursday evening: "I am the elected president of the Republic of Ivory Coast."
"The Constitutional Council has abused its authority, the whole world knows it, and I am sorry for my country's image," he said.
There have been dramatic scenes since Sunday over the declaration of the results.
On Tuesday, Mr Gbagbo's representative in the electoral commission tore up the first batch of results as the commission's spokesman was about to announce them.
The electoral commission head, Youssouf Bakayok, then went ahead with an announcement on Thursday, speaking under armed guard at a hotel rather than from the commission's headquarters, declaring Mr Ouattara the winner with 54% of ballots cast.
Not long afterwards, Mr N'Dre said that, as the announcement had come after Wednesday's legal deadline, those results were "null and void".
The presidential poll was intended to reunify the world's largest cocoa producer after a civil war in 2002.
The two candidates represent the two sides of the north-south divide that exists religiously, culturally and administratively, with the northern half still controlled in part by the former rebels.
In a statement, the country's Prime Minster Guillaume Soro said the voiding of votes "threatens the ideal of reunifying the country".
The BBC's John James in Abidjan, the country's main city, says the Constitutional Council's decision has come as a shock to many, especially the opposition.
Youths from the opposing camps took to the streets in Abidjan and other towns, throwing stones and burning tyres.
The military has closed the borders and international news sources are suspended. An overnight curfew is in place.
Both the army and UN peacekeepers have been patrolling the streets of Abidjan since Sunday.
At least four people have been killed in election-related clashes in Abidjan this week.