Opposition leader Ahmad Wald Dadah said "first indications point to massive fraud across the country", after casting his vote in the capital, Nouakchott, on Friday
He added that "stuffing of ballot boxes began at 7:00 am", when polls opened.
Another candidate, Muhammad Khuna Wald Hidala, who was briefly detained by police on Thursday and accused of plotting a coup, said there had been "all sorts of intimidation" of voters.
But the incumbent president, Muawiya Wald Sidi Ahmad Taya, said that voting was "totally transparent".
The election would "no doubt be considered an example for those who are interested in the establishment of democracy in the world", he added.
The arrest of Wald Hidala prompted the International Federation of Human Rights Leagues to call for the poll to be suspended.
They argued there is a law which says polls must be put off if a candidate is prevented from standing by illness, death or arrest.
Opposition leader Wald Dadah
was president from 1980-4
Although Mauritania is one of the world's least developed nations, voters are using a high-tech identity card when they cast their ballots to prevent fraud.
However, international observers have not been invited to witness the vote.
In the run up to the elections, New York-based group Human Rights Watch (HRW) said the government's harassment of opposition figures undermined any chance of free and fair elections.
It said from late April, Mauritanian security forces arrested dozens of religious leaders, opposition politicians and social activists, allegedly in a campaign to crack down on terrorist movements.
Peter Takirambudde, of HRW, said: "It seems that this is yet another example of a government opportunistically using the language of counter-terrorism to crack down on legitimate dissent."
The rights group added the Mauritanian government has restricted freedom of expression and association for many years.
Non-governmental organisations and the media are repeatedly shut down, refused access to public forums, or censored for expressing opinions critical of government policies.
President Wald Taya has been
accused of harassing opponents
Mauritania's presidential election feautures six candidates.
Wald Hidala was president from 1980 to 1984, after a military junta overthrew Moktar Ould Daddah, the father of Mauritanian independence, in 1978.
He was in turn ousted in 1984 by Wald Taya who has been president ever since, although elections in 1992 were tainted by fraud charges and in 1997 boycotted by the opposition.
Besides outgoing President Wald Taya and former president Wald Hidala, four other candidates are standing, one of them a woman.
They include Massoud Wald Bilkhair, the first descendant of slaves to seek the highest office in the Muslim country, a vast stretch of desert with a population of less than three million.
If none of the candidates wins more than 50% of the vote, there will be a second round of voting in two weeks.