The Group for the Defence of the Rule of Law was formed to "work towards preserving the rule of law, civil peace and the defence of democratic rights", spokesman Muhammad Walad Dahani said on Saturday.
The group demanded fair trials for the suspects arrested after a June 2003 coup attempt - but also called for justice and compensation for the victims of the violence, which claimed 15 deaths and injured 68.
Walad Dahani accused several rights groups, including the Paris-based International Federation for Human Rights (FIDH), of "spreading rumours and lies" about alleged mistreatment of suspects held over the coup.
He charged that the rights groups "paid little attention to the damage caused to the institutions of the state and to innocent civilians".
Mauritanian authorities have recently arrested a number of people, including the alleged mastermind of the June 2003 coup, who authorities say may also have been behind another attempt last month to overthrow President Muawiya Walad Sidi Taya.
The government has blamed the coup attempts on a group of disgruntled military officers angry at Walad Taya's pro-western policies and diplomatic recognition of Israel.
Critics of Taya say the coup plots are staged attempts by the government to ease the fears of investors, and prove that it has control over its wide-ranging population.
Rights groups have also expressed concern that some people who have been arrested are being held in secret locations and are being tortured.