Defying international calls to step down, Mr Gbagbo has sworn himself in for a new term and named a government, despite provisional results that gave Mr Ouattara a near ten-point margin.
Mr Ouattara has taken a presidential oath in a rival ceremony and refused to back down in a power struggle that risks sending the West African nation, the world's top cocoa grower, back into conflict eight years after civil war split it into north and south.
The UN mission chief in the country, YJ Choi, rejected accusations from the Gbagbo camp that he had meddled in internal affairs by declaring Mr Ouattara the winner, and said an Ivorian Constitutional Council move to annul Mr Ouattara's victory was "not based on facts".
"I remain absolutely certain that I have found the truth concerning the will of the Ivorian people. The people have chosen one person … Mr. Alassane Ouattara with an irrefutable margin," he said.
Mr Gbagbo's allies argued intimidation and fraud skewed the vote in the rebel-held north, a complaint the Constitutional Council - run by a staunch Gbagbo ally - upheld and used as grounds for cancelling hundreds of thousands of votes.
But Mr Choi said there had been fewer recorded acts of violence in the north than in pro-Gbagbo western districts, and noted that even if these and other accusations were true, in the final tally Mr Ouattara still had secured a clear majority.