According to partial results released early on Saturday by the interior ministry, Ahmad Taya was well ahead of the five other candidates, securing between 64 and 65% of the vote.
These were the figures after counting from just over 300 of the 2258 voting stations, director of political affairs Sidi Yeslem Wald Amar Cheine told reporters.
Next came former president Muhammad Khuna Wald Hidala with 20% followed by Ahmad Wald Dadah and Masaud Wald Bilkhair both with 6%, according to the partial figures.
The two minor candidates, Aisha Bint Jidanah and Moulaye Eal-Hasan Wald Jiad, shared the remaining votes, Amar Cheine said.
A source close to his campaign management said Taya, who has been in power since 1984, was ahead in all the northwest African country's 13 regions.
From the start of voting on Friday, the main opposition candidates had claimed fraud and intimidation of voters.
"Fraud has reached such dimensions that this election cannot be validated"
Shaikh Wald Horma,
Hidala's deputy campaign manager
At the end of the polls, their campaign managers said there was "no doubt" that Taya would win, with Hidala's deputy campaign manager Shaikh Wald Horma declaring: "Fraud has reached such dimensions that this election cannot be validated."
But Taya's campaign manager, Interior Minister Hamad Wald Muhammad, rejected the charges and said Daddah's claims of ballot-box stuffing were "lamentable".
Shortly after voting ended, Taya's spokesman Muhammad Vall Wald Bilal said the election had gone well, despite the predictions of the "prophets of doom."
The outgoing president was well placed to win in the first round, he added.
As voting ended, security forces closed the streets leading to the presidential palace.
Elections in 1992 were tainted by fraud charges and in 1997 boycotted by the opposition.