Officials announced Wald Taya has been re-elected the West African country's president with 66.7% of the votes. He has been the president since 1984.
But the victory was marred by shrill allegations of fraud and intimidation.
Campaigning staff of Muhammad Khuna Wald Hidala, the principal opposition challenger, said he "had decided to take shelter from the arbitrary conduct of the government while waiting to resume his public activities."
Rumours had been rife since Friday that Hidala had left for an undisclosed destination to avoid arrest.
Hidala and some of his supporters had been arrested on the eve of the poll on charges of plotting a coup, but were later freed.
But opposition activists maintained the presidential polls had been rigged and that there was "no doubt" that the incumbent president would win.
"Fraud has reached such dimensions that this election cannot be validated," one prominent opposition member alleged.
Hidala's supporters have dismissed the election as "invalid, null and void" because of massive fraud and intimidation.
Though one-time comrades, Wald Taya and Hidala have been bitter rivals since Taya ousted him as the country's president in 1984 after a four-year tenure.
Hidala and Wald Taya were together in 1978 in overthrowing the government of Muktar Wald Daddah, considered Mauritania's father of independence.