Mauritania's ruling party candidate has won the first democratic transition of power since independence in 1960.
Mohamed Ahmed Ould Ghazouani, a close ally of outgoing President Mohamed Ould Abdel Aziz, won with 52% of votes.
Four of the opposition candidates have rejected the results, which are expected to be submitted to the constitutional council for validation.
Among them was the nearest rival, anti-slavery campaigner Biram Dah Abeid, who gained 18.5% of votes.
"This seems like a coup d'etat," said Mr Abeid, adding that those contesting the results were "united".
BBC West Africa correspondent Louise Dewast said the electoral commission has repeatedly rejected allegations that they are biased in favour of the governing party and said the vote had gone smoothly.
Although largely peaceful, there were some protests earlier on Sunday in coastal areas.
On Friday, Mauritania's press authority said it had received no complaints about the coverage of the campaign.
The outgoing president stepped aside after two five-year terms leading the mainly desert country with a population of less than five million.
Under his leadership the economy has grown but the issue of slavery persists. Mauritania became the final country in the world to formally abolish slavery in 1981, but according to human rights groups, tens of thousands of black Mauritanians are forced into domestic slavery by people of Arab or Berber descent.
Criminal laws allowing slaveholders to be prosecuted were passed in 2007, but have yet to be fully and effectively enforced.
After Mauritania achieved independence from France in 1960, the country's first president held power for 18 years before being ousted in a military coup. More coups followed in 1984, 2005 and 2008.