Shaikh Muhammad al-Didaw's was the latest in a string of detentions of activists who have publicly called for national conciliation among political forces in the country, Aljazeera's correspondent said on Monday.
Head of the Mauritanian Islamist Movement (MIM), al-Didaw's arrest follows those of fellow members Jamil Walad Mansur and Mukhtar Musa.
Sources close to the government said all three were taken into custody in the city of Roso on the border with Senegal after being linked to a failed coup masterminded by Captain Salih Walad Hananna.
At least two attempts to oust President Muawiya Walad Taya have been made in the last 15 months.
In August, Mauritanian troops were confined to barracks in the capital, Nouakchott, after unconfirmed reports of a foiled putsch.
An opposition deputy and a human rights lawyer on 10 August said there was talk of more than a dozen army officers having been arrested at the time.
The apparent attempt to kill the president took place at the airport as he was leaving for France. There was no official confirmation.