Arrested on 2 November 2004 in Nouakchott, Muhammad Didaw, Jamil Walad Mansur and Mukhtar Walad Muhammad Musa were released on Sunday.
Attorney-general Muhammad Walad Umar said all three had been granted provisional liberty, but provided no additional details.
In an earlier press statement Mansur and Musa - both members of an Islamist movement in the Mauritanian capital - had vowed their protest would be of "unlimited duration" and end only when their cases were handled correctly.
The suspects had also pointed to the slowness and legal irregularities with which their cases were being handled.
The pair were originally seized for circulating evidence on internet sites and an Arabic television station, that police tortured suspects in connection with investigations into alleged attempts to overthrow President Muawiya Walad Taya.
The government has charged that photographs posted on the websites were faked and were intended to incite public unrest and threaten public security.
Mauritania wrapped up its largest-ever trial earlier this month, issuing guilty verdicts to 84 convicted coup plotters and acquitting more than 100 other defendants including a former president accused of bankrolling three plots to overthrow Taya.
Amnesty International alleges that torture is widespread in Mauritania.