Ivory Coast President Alassane Ouattara spent the past five months barricaded inside a hotel, surrounded by troops loyal to Laurent Gbagbo, who used the army to terrorize the population. Gbagbo was removed militarily last month and is now under house arrest in a remote town 420 miles (700 kilometers) north of Abidjan.

The price of installing the country's democratically elected leader was steep. More than a 1,000 civilians were killed, first by the army controlled by Gbagbo and later by a former rebel group allied with Ouattara that seized control of the country and toppled Gbagbo.

Hours before the ceremony Friday, the U.N. human rights office in Geneva announced their investigators were headed to a soccer field in an Abidjan neighborhood believed to be the site of a new mass grave.

Inside the presidential palace, Ouattara stood a story above the basement where reporters found more than 500 boxes of 121mm artillery rockets in the days after Gbagbo's capture, weapons that are capable of leveling buildings and which left the streets of Abidjan strewn with corpses last month.

Ouattara raised his right hand and swore to protect the constitution in front of Paul Yao N'Dre, one of Gbagbo's closest allies who months earlier had used his position on the country's highest court to overturn Ouattara's victory.

"I solemnly swear on my honor to respect and faithfully defend the constitution," Ouattara said. He also praised the constitutional council headed by N'Dre _ a longtime friend of Gbagbo _ for finally accepting its error.

"I salute first of all the members of the constitutional council for the courageous decision they have taken in saying what is right," he said. "This decision will allow them to reconcile themselves with their oath and with their conscience. It re-establishes the truth of the ballot box and sets itself right with the people of Ivory Coast who massively participated in the Nov. 28 election," he said.

According to international observers, Ouattara won the November election with 54 percent of the vote. Gbagbo loyalists on the country's electoral body tried to stop the results from being published.

Once they were public, N'Dre went on TV to announce that he was invalidating ballots from nearly all of Ouattara's strongholds, claiming fraud. In all, more than half a million votes were chucked out in order to declare Gbagbo the winner.

Gbagbo held a shotgun inauguration, where he was sworn in by N'Dre in a ceremony that was boycotted by nearly the entire diplomatic corps. On Friday afternoon, Ouattara wearing a sharp black suit and his French wife looked out across rows of diplomats, as well as the members of his government.

Earlier Friday morning, investigators headed to a soccer field where Red Cross workers had received reports of a mass grave, said Hamadoun Toure, spokesman for the U.N.

"We are told that there is a vast field that is used to play soccer. It is now an open-air cemetery," he said.

The soccer field is located in Yopougon, an area of Abidjan that had voted in large numbers for Gbagbo. His militias are believed to have taken cover in Yopougon, and the neighborhood was the scene of pitched battles until Thursday, when Ouattara's military spokesman announced that the area had been brought under control.

Toure said it's not clear if the dead were killed by Gbagbo's forces, or if they are Gbagbo supporters slain in reprisal killings by forces loyal to Ouattara. Human rights groups have detailed massacres by the forces backing Ouattara, who swept the country coming in from the north, east and west.

In the village of Duekoue, the United Nations estimates that more than 400 people were killed and U.N.

investigators saw another 40 bodies in one village and 60 in another as they flew over the area in a helicopter.

Ouattara has promised an investigation into killings by both sides.

Gbagbo is under house arrest in Korhogo, a town in the interior. His French lawyers were expected to travel to Ivory Coast this week to accompany him to an interview with the police, which has been rescheduled several times. He is facing possible charges of war crimes and crimes against humanity committed by the army under his control in the postelection period.