The leading opposition candidate in Mauritania's presidential election has been arrested on the eve of the vote. Mohamed Khouna Ould Haidallah was taken from his home to an unknown location. Earlier this week, police arrested two sons of Mr Haidallah in a raid on his home. On Wednesday, police used tear gas to break up a demonstration against the arrests in the centre of the capital, Nouakchott. **Tear gas*** Mr Haidallah's spokesman, Ely Ould Sneiba, said: "They [the police] went to his house and forced him to go with them." He said he did not know why Mr Haidallah had been arrested, Reuters news agency reported. A day earlier, supporters of Mr Haidallah and two other opposition leaders - Ahmed Ould Daddah, the half-brother of Mauritania's first president, and Messaoud Ould Boulkheir, the first descendent of slaves to run for president - led a protest against the arrest of Mr Haidallah's sons. A communique issued by the director of security in Nouakchott said many groups supporting Mr Haidallah, including Islamists, were planning violence if they lost the ballot. Mr Haidallah denied the allegations and said that he had a permit for one of two weapons found at his house, while the other dated from the 1970s. **'Climate of harassment'*** Mauritanian President Maaouiya Ould Taya is seeking re-election six months after the army put down a coup attempt. 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. **PHOTO CAPTION*** Mauritania's President Maaouya Ould Sid'Ahmed Taya speaks at an electoral rally in a stadium of the capital Nouakchott, November 5, 2003. (REUTERS/Luc Gnago)