Gen Abdelaziz ousted Mauritania's only democratically elected leader
The head of Mauritania's electoral commission has resigned because he has doubts over the "reliability" of last week's presidential vote.
Sid'Ahmed Ould Deye said that the complaints he had received had sown doubts in his mind.
The opposition said the poll was rigged and lodged complaints.
Later, the Constitutional Court confirmed that the election was won by Gen Mohamed Ould Abdelaziz, who took power in a coup last year.
On Sunday, the election commission gave victory to Gen Abdelaziz, stating he had won the poll outright with 52% of the vote, and put the turnout at 61%.
Claims of fraud
Mr Deye said he had resigned as a matter of "conscience".
"The complaints I received, as well as the contents of the challenges sent to the Constitutional Court, have sown doubts in my mind about the reliability of the election," he said.
Three of the opposition candidates had lodged complaints with the court, alleging massive fraud.
However, shortly after his resignation, the court, which investigates complaints and has the power to declare the vote invalid, confirmed the results.
It dismissed the opposition's claims of fraud due to lack of evidence, and said the elections had been "clean and transparent".
General Abdelaziz has denied that the vote was rigged.
Overseas observers praised the poll's conduct but said the opposition challenges should be investigated.
The general staged a coup in August 2008, ousting Sidi Ould Cheikh Abdallahi - Mauritania's first democratically elected leader who had been in power for less than a year-and-a-half.