Turn-out has been high in the capital, Nouakchott

Men wearing long, pale blue or white robes formed separate queues to women wearing more colourful gowns.

A BBC reporter in the capital, Nouakchott, said turn-out was high and voting peaceful, but the opposition claims the polls are being rigged.

Armed police were patrolling the streets as voting took place, the BBC's Pascale Harter says.

Casting his vote, President Maaouiya Ould Taya said no country had more transparent elections. The polls see the first woman and the first descendant of slaves to run for the presidency.

Supporters of opposition candidate Ahmed Ould Daddah in the east of Mauritania say polling stations were closed at 0900, just two hours after they opened, and that the opposition ballot papers were not available for voters.

These reports have not yet been independently confirmed.

No observers

Mr Taya, who arrived at a Nouakchott polling station in a Mercedes flanked by 14 motorcycle outriders, wearing a pinstripe suit and gold-rimmed sunglasses, said the election was fair.

"The ballot box is transparent, the voting list is on the Internet... and the cards can't be falsified," he declared.

MAURITANIA Dominated by light-skinned Arabic-speakers (Moors) Campaigners say slavery still practised Black Africans complain of discrimination Mostly Sahara desert Islamic Republic Recognises Israel The vote in pictures Mauritania's nomadic elections

Foreign election observers have not been invited and the polls are being organised by the interior ministry, which has rejected the fraud allegations.

One voter in the capital told the BBC's Network Africa: "It is not the voting that is a problem but the counting".

On the eve of the poll, Islamist opposition candidate Mohamed Khouna Ould Haidalla was detained, in what he said was "just part of the ongoing intimidation".

Police are believed to be still holding two sons of Mr Haidalla and a number of his close associates.

State Prosecutor Mohamed Elgheiz Ould Oumar accused the group of planning to "change the government by violence and insurrection" and warned that Mr Haidalla could be re-arrested "at any time".

The accusation has been denied by the candidate's team.

'Climate of harassment'

President Taya is seeking re-election six months after the army put down a coup attempt.

Power has never changed through the ballot box in Mauritania

Mr Taya seized power in 1984 in a coup from Mr Haidalla who had been put in office by a coup himself in 1980.

He went on to win elections in 1992 and 1997, amid allegations of fraud and opposition boycotts.

In September, the New York-based group Human Rights Watch warned of a "climate of harassment of opposition members" following the arrest of several opposition activists and Islamists.

Racial tensions

Mauritania is divided between three main groups: light-skinned Arabic-speakers; the descendents of their slaves who have adopted their language and culture and black Africans who are culturally closer to the people of neighbouring Senegal and Mali.

Haidalla is President Ould Taya's strongest contender

An Islamic state, it is nevertheless one of the handful of Arab states to recognise Israel and President Ould Taya has pursued an alliance with the United States

Campaigners say slavery is still practised in the vast desert state despite its official abolition in 1981 and one of the candidates is Massaoud Ould Bolkheir, the first descendant of slaves to seek the highest office.

Mr Bolkheir says his aim is to "introduce legislation forever banishing slavery, racism, tribalism and all the caste ideologies found in Mauritania".

Another running this year is the first woman candidate, Aicha Mint Jeddane.

Results are expected to be known by Saturday.