The 14 people, including a university lecturer, two imams, and students, were arrested at their homes on 25 April and held at the national police headquarters in the capital, Nouakchott.
After investigation, they were released on Friday mainly due to lack of evidence, a police official said.
The group includes lawyer and rights activist Mohammed Ahmed Ould Hajj Sidi, who was on hunger strike.
Another group of 37, also arrested last month, faced similar allegations of forming a terror group, their lawyers said. They could either be officially charged or freed.
The group includes Cheikh Mohamed Hassan Ould Dedew, spiritual leader of many of Mauritania's Islamic radicals, and Moktar Ould Mohamed Moussa, who previously served as ambassador to the United Arab Emirates.
The authorities have increased arrests recently to curb what they say are illegal terrorist activities in the Sahara Desert nation.
Early this month, seven members of a clandestine group allegedly trained by the Salafists, which Western authorities have linked to the al-Qaida network, were charged and sent to jail.
Islamist leaders in Mauritania have staunchly opposed President Maaoya Sid'Ahmed Taya - who has survived several coup attempts during his 20-year leadership and cracked down ruthlessly against opponents, jailing scores of people.
The country's beleaguered opposition says ruling-party officials unfairly tar them with the terrorist tag in a bid to undermine their efforts to organise.