The move by Adrien Houngbedji, who scored 36 percent against Boni Yayi’s 53 percent according to results approved by the constitutional court, came after the regional ECOWAS bloc said it was worried by tension in some parts of the country.
Cotton-producer Benin has managed to avoid much of the instability seen in the region but March 13’s election had to be delayed twice due to opposition complaints that hundreds of thousands of people had been left off the election register.
International observers from the African Union and other have said the vote was credible despite some failings, but Houngbedji rejected the outcome, saying it had been rigged by widespread ballot-stuffing.
“I won this election. I am the elected president of the people of Benin,” he said in a declaration made before supporters and media.
“The results proclaimed by the Constitutional Court are merely the latest proof of the big plot being led against democracy in this country,” he said.
Benin with its population of 9 million has won international praise for avoiding the kind of turbulence that has followed recent elections in Guinea and Ivory Coast.
Houngbedji’s move is reminiscent of a move in January by Gabonese opposition leader Andre Mba Obame to swear himself in as president in a mock ceremony, contesting the legitimacy of President Ali Bongo Ondimba after a disputed 2009 election.
Gabonese authorities have reacted by dissolving Obame’s National Union (UN) party and countries including France and the United Stated have tended to ignore Obame’s step and continue to recognise Bongo as president.
Earlier, president of Benin’s constitutional court said Yayi was re-elected with 53.13 percent against 35.64 percent for Houngbedji, giving him a first-round victory.
“Boni Yayi won an absolute majority of votes required to be declared elected,” Robert Dossou said earlier on Monday.
Other opposition candidates have five days to contest the court’s decision before the results become final.
Nigeria’s President Goodluck Jonathan, who is also chairman of ECOWAS, visited the neighbouring country on Friday and urged the candidates to respect the results and resort to legal means in case of disputes.