President Ould Taya is ready for Friday's vote

Hundreds of supporters of three different opposition candidates had gathered in the centre of the capital, Nouakchott, for a joint march.

The march was called in protest at a police raid on the home of the main opposition candidate on Monday.

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

The police said they had not been notified about the demonstration.

'Harassment'

They arrested the second son of opposition leader Mohamed Khouna Ould Haidallah, after arresting his first son when they raided his house and found weapons.

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The march was also backed by supporters of two other opposition leaders: Ahmed Ould Daddah, the half-brother of Mauritania's first president, the late Moktar Ould Daddah; and Messaoud Ould Boulkheir, the first descent of slaves to run for president.

A communique issued by the director of security in Nouakchott said many groups supporting Mr Haidalla, including Islamists, were planning violence if they lost the ballot.

Mr Haidalla denied the allegations and said that he had a permit for one of the two weapons found at his house, while the other dated from the 1970s.

Mr Ould Taya seized power in 1984 and won elections in 1992, amid allegations of fraud, and 1997, when most opposition parties refused to participate.

In September, 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.