At least nine campaigners including Biram Ould Dah Ould Abeid, President of the Initiative for the Resurgence of the Abolitionist Movement (IRA), are being held in the southern city of Rosso after they were detained this week.
"Anti-slavery activists are subject to never ending harassment and intimidation in Mauritania," Gaetan Mootoo, a west Africa researcher for Amnesty said in a statement.
IRA and other rights organisations have been holding rallies across the country and dozens of campaigners demonstrated in the capital Nouakchott on Tuesday and yesterday, according to an AFP reporter.
"Their actions are either prohibited or severely repressed and they are frequently arrested. This general clampdown must stop as it is a clear violation of the rights to freedom of peaceful assembly and association," said Mootoo.
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Police closed IRA's headquarters in Nouakchott yesterday, a government source told AFP, adding that it was considered a "non authorised" group of extremists.
The arrests are part of an increasing clamp down on anti-slavery activists in Mauritania, according to Amnesty, which added that four IRA members were arrested in Nouakchott's largest mosque in October.
Mauritania has agreed to adopt a roadmap prepared by the United Nations High Commissioner for Human Rights for eradicating the slave trade, which campaigners say remains widespread in the west African nation.
The country was the last in the world to abolish slavery, in 1981, and since 2012 its practice has been officially designated a crime, but campaigners say the government has failed in the past to acknowledge the extent of the trade, with no official data available.