Abidjan, Ivory Coast (CNN) -- First came a political row over the rightful winner of Ivory Coast's presidential election. Saturday, two men laid claim to the West African nation's highest post.
Incumbent Laurent Gbagbo defied international appeals to step aside and was sworn in as the new president in a formal ceremony inside the presidential palace that was broadcast live on television.
Less than an hour-and-a-half later, his rival, Alassane Ouattara, told reporters that he, too, had taken the oath of office and asked Prime Minister Soro Guillaume to form a new government.
Abidjan, under a 7 p.m. curfew, saw an eerie calm fall Saturday evening. But the political chaos heightened fears that the Ivory Coast -- known as Cote d'Ivoire in French -- would once again plunge into the unrest and bloodshed suffered after a civil war broke out in 2002.
The Constitutional Council declared Gbagbo the winner Friday, invalidating earlier results from the Independent Electoral Commission which handed Ouattara the victory with 54.1% of the vote.
The Constitutional Council said Gbagbo had won the election with 51.45% of the vote to Ouattara's 48.55%. It tossed out votes it said were marred by fraud in northern regions that were considered Ouattara strongholds.
It was the job of Y.J. Choi, the special envoy in the Ivory Coast of U.N. Secretary-General Ban Ki-moon, to review and sign off on the results. Choi said that, even if Gbagbo's complaints were taken into consideration, Ouattara was the winner.
"Having evaluated all the tally sheets, 20,000 of them yesterday evening, we are in a position to know what happened really," Choi said in a telephone interview from Abidjan. "With absolute certainty, we know that Ouattara won the election."
With his credibility in question, Gbagbo defied calls from international leaders to respect the will of Ivorian voters by taking the oath of office in front of a room full of supporters and military commanders.
"These past days I have noticed serious cases of interference," Gbagbo said. "The sovereignty of Cote d'Ivoire, is not negotiable. My responsibility is to defend it."
Meanwhile, the African Union stepped into the fray and said it would entrust former South African President Thabo Mbeki to help determine a rightful end to the dispute.
"The purpose of this mission is to facilitate the rapid and peaceful conclusion of the electoral process and the efforts to find a way out of the crisis in (the Ivory Coast) on the basis of relevant (African Union) decisions and instruments and through consultations with the concerned actors," said a statement from the intergovernmental organization.
French President Nicolas Sarkozy appealed for calm and urged military and civilian officials to respect the will of the people.
U.S. President Barack Obama warned Gbagbo: "The international community will hold those who act to thwart the democratic process and the will of the electorate accountable for their actions."
Ouattara, a former economist for the International Monetary Fund who served as prime minister, had been banned from previous races.
Gbagbo's critics said the incumbent stoked tensions by accusing Ouattara of masterminding the civil war. Ouattara has denied the allegation.
Once a prosperous nation and a driving force in West Africa, the Ivory Coast spiraled downward into instability after fighting erupted between the government-held south and discontented Muslim rebels living in the north. Thousands of people died in the conflict.
Ouattara enjoys popular support in the rebel-held north and now, with both candidates claiming they are president, the potential exists for more bloodshed. The streets of Abidjan have already seen violent clashes in the past few days.
Journalists Francois Xavier Menage and Eric Agnero contributed to this report.