Laurent Gbagbo took office before a room full of supporters at the Presidential Palace in Abidjan, CNN reported. Hours later, rival Alassane Quattara also took an oath as president at a hotel, the BBC reported.
The Constitutional Council declared Friday that Gbagbo took 54 percent of the vote to opposition candidate Ouattara's 46 percent.
On Thursday, the Independent Electoral Commission declared Ouattara the winner with 54 percent, but the court invalidated votes it said were tainted by fraud in Ouattara's northern strongholds.
Y.J. Choi, a special envoy from U.N. Secretary-General Ban Ki-moon who examined the results, said that, even with Gbagbo's complaints taken into account, Ouattara was the victor.
"Having evaluated all the tally sheets, 20,000 of them yesterday evening, we are in a position to know what happened really," Choi told CNN. "With absolute certainty, we know that Ouattara won the election."
U.S. President Barack Obama and French President Nicolas Sarkozy have congratulated Ouattara.
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."