Visitors were allowed to visit 32 soldiers and army officers who were arrested by by the former government on suspicion of having participated in the 2003 coup attempt, which came close to toppling the ousted president Maaouya Ould Sid'Ahmed Taya, a civic group said on Tuesday.
Army officers who toppled Taya this month are keen to bolster public support for their coup, welcomed by many in the West African country who resented his authoritarian rule and repression of Islamist leaders.
Visitors flocked to the main prison in the capital, Nouakchott, on Monday, where relatives have staged a sit-in to demand access to prisoners since the military officers seized power on 3 August and promised elections within two years.
"We don't just want to visit them, we want them to be freed," said Fatou Mint el-Kory, president of a group representing the detainees relatives.
"They are political prisoners. I am appealing to everyone to support us so that human rights and justice triumph," she said on Tuesday.
The army rulers ordered the release of about 20 Muslim activists four days after their coup.
The freed men were members of a group of 51 people arrested by security forces since April in a clampdown on Muslim activists.
"I noticed that there were certain problems in the prisons and I have given instructions for them to be addressed," said Justice Minister Mahfoudh Ould Bettah after visiting detention centres on Monday.
Taya had turned Mauritania, which straddles black and Arab Africa, into one of the most repressive countries towards Islamist movements, especially after narrowly surviving a coup attempt in June 2003.
The detained Muslim activists had been accused by Taya's government of colluding with the Algerian-based Salafist Group for Preaching and Combat (GSPC), a movement allied to al-Qaida.