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The 62-year-old former head of the domestic security service made the claim in the early hours of Sunday in the presence of current president Mohamed Ould Abdel Aziz, his supporters and journalists.

A source at the Independent National Electoral Commission (CENI) said Ghazouani had won 50.56 percent of the votes after 80 percent of the votes had been counted following Saturday's election.

"There is only 20 percent left (to count), but that will not change the final result," Ghazouani reportedly told supporters.

The CENI source said leading opposition candidates Sidi Mohamed Ould Boubacar, a former prime minister, and Biram Ould Dah Ould Abeid had each garnered about 18 percent with the count continuing.

Both men had complained of balloting irregularities and the expulsion of representatives from some polling stations. However CENI said no major problems had been reported.

Ghazouani - who campaigned on the themes of continuity, solidarity and security for the vast largely desert nation - served as Abdel Aziz's chief of staff from 2008 to last year.

The outgoing president is a general who originally came to power in a 2008 coup, won elections a year later and was again elected in 2014 in polls boycotted by the opposition.

Abdel Aziz gave full backing to his loyal lieutenant.

"There's only two choices - either going backwards, towards extremism, waste and instability, or your candidate, who will continue what has been achieved to build a stable and developed state," he declared at the rally. 

The ballot is the first in Mauritania's coup-strewn history that looks set to see an elected president complete his mandate and transfer power to an elected successor, but the opposition has raised concerns that it could perpetuate a government dominated by military figures.