ABIDJAN, Ivory Coast - The two candidates in Ivory Coast's disputed presidential election took dueling oaths of office Saturday after each claimed victory, as the political crisis spiraled out of control and renewed unrest in this country once split by civil war.

Incumbent President Laurent Gbagbo defied calls from the United States, France, and the United Nations to concede defeat, wrapping himself in the Ivorian flag as he was sworn in for another term. Hours later, opposition candidate Alassane Ouattara announced that he, too, had taken his own oath.

Saturday's developments leave Ivory Coast with two men who both claim to be president, further inflaming the political chaos in the West African nation whose once-prosperous economy was destroyed by the brief 2002-03 civil war.

President Obama and French President Nicolas Sarkozy say that Ouattara is the rightful winner and that his victory must be acknowledged. The top U.N. official in Ivory Coast also stands by results released Thursday by the country's election commission that put Ouattara ahead.

U.N. Secretary-General Ban Ki-moon expressed "deep concern over the continuing standoff," his spokesman said in a statement Saturday.

Prime Minister Guillaume Soro, a former rebel leader who had served in a unity government with Gbagbo since a 2007 peace deal, said Saturday that he was backing Ouattara, who is wildly popular in the formerly rebel-held north. Ouattara said his first act as president was to reappoint Soro.

"These last days have been difficult, but I can tell you now that Ivory Coast is in good hands," Ouattara said.

At his swearing-in, Gbagbo renewed allegations that his supporters had been intimidated in the north, repeating the rationale used by the country's constitutional council to throw out a half-million ballots that were cast in Ouattara strongholds.

"You think that you can cheat, stuff ballot boxes, and intimidate voters and that the other side won't see what is going on," Gbagbo said.

Ivory Coast's election was meant to restore stability in what was once one of the most affluent countries in Africa. Instead, the election has cast a growing shadow as the country now faces two political rivals who claim to be leading the country.

On Saturday, Ouattara supporters took to the streets, burning tires and a table in one neighborhood.

"The risk of violence between supporters of the two parties, as well as repression by Ivorian security forces against real or perceived supporters of Ouattara, is very high," Corinne Dufka, senior West Africa researcher at Human Rights Watch, said in a statement.

Gbagbo's five-year mandate expired in 2005, and the country's first election in a decade was delayed several times.

The election was held in October, but then headed to a runoff vote last Sunday. The country's election commission announced Thursday that Ouattara had won. However, new results released Friday on national television by a Gbagbo loyalist who heads the constitutional council said the incumbent had been reelected.

Gbagbo says he is the rightful winner of the runoff vote, citing the Ivorian Constitution, which gives ultimate authority on the issue to the country's constitutional council, which declared him the winner.

However, Ouattara points to the 2007 peace deal, which states that the United Nations must certify the election results. The United Nations maintains the vote was credible, and that Ouattara won the presidential election.