A prominent Mauritanian anti-slavery activist has been taken into custody before legislative elections next month in what rights groups fear is a politically-motivated move to silence opposition.

Biram Dah Abeid, who heads Mauritania’s Initiative for the Resurgence of the Abolitionist Movement, was arrested early on Tuesday morning at his home in the capital, Nouakchott, and imprisoned in the southern part of the city.

He has not yet been charged with any crime, according to the Unrepresented Nations and Peoples Organization (UNPO).

Abeid informed colleagues of his arrest on WhatsApp, describing the moment when police knocked on his door and told him there was “an order from above” that he should “follow them to the police station”.

Conjecturing that his arrest may be related to “popular activities” he organised a few evenings earlier, Abeid concluded: “I know it’s linked to the fact that the electoral commission should be giving us the final list today of all the candidates running for office.”

Legislative elections will be held on 1 September, with Mauritanians set to vote for members of parliament as well as regional and local councillors. Abeid, along with other anti-slavery activists, some of them former slaves themselves, are due to run in the polls against the majority Arab-Berber government.

Although Mauritania made slavery illegal in 1981 – the last country in the world to do so – tens of thousands of people, mostly from the minority Haratine or Afro-Mauritanian groups, still live as bonded labourers, domestic servants or child brides. Local rights groups estimate that up to 20% of the population is enslaved, with one in two Haratines forced to work on farms or in homes with no possibility of freedom, education or pay.

Abeid, a Haratine and himself the son of a slave, has vowed to oust the incumbent president, Mohamed Ould Abdel Aziz, in national elections next year. Aziz came to power in a 2008 coup and has since dismantled the senate in what critics see as a bid to broaden his powers.

Abeid founded the Initiative for the Resurgence of the Abolitionist Movement in 2008, and has been arrested many times for his anti-slavery activism. He was most recently released in 2016, after spending 20 months imprisoned on charges related to “inciting trouble” and belonging to “an unrecognised organisation”, according to UNPO.

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Karine Penrose-Theis, Africa programme manager at Anti-Slavery International, said his recent arrest was “very worrying”.

“The Mauritanian government has a long track record of cracking down on anti-slavery activists and, given that he was due to stand in the parliamentary election in the autumn, one can’t help but worry,” she said.

“Rather than cracking down on activists, it’s time that the Mauritanian government came clean on slavery that still affects thousands of people in the country and start tackling it with the seriousness it deserves.”