Sid'Ahmed Ould Dey told journalists in the capital that while his commission ruled the elections to be free and fair, he had questions about the declaration of a landslide win for coup leader Mohamed Ould Abdel Aziz.

He offered no proof of irregularities in the election, which international observers said were free of major problems.

"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," Dey said. "It is for that and not to be in contradiction with my conscience that I decided to present my resignation."

He added: "I do not have material proof of fraud."

Aziz won 52 percent of the vote, avoiding a runoff, according to an announcement by Mauritania's interior minister Sunday.

The country's lead opposition group accused Aziz of committing an "electoral coup."

On Tuesday, three candidates appealed the poll results. Veteran opposition leader Ahmed Ould Daddah, Parliament speaker Messaoud Ould Boulkheir and Ely Ould Mohamed Vall denounced the weekend election as a "massive fraud."

The constitutional court has until Wednesday to rule on the appeal.

The opposition has made several complaints against Aziz's group, including allegations that it handed out ballots pre-marked in his favor and paid voters to cast them.

Aziz came to power in an August 2008 coup, but resigned from the army and from his post as junta leader to run in the July 18 elections.