More than 100 members of the Initiative for the Resurgence of the Abolitionist Movement in Mauritania (IRA) protested in the capital for the release of the organisation`s leader and another employee, the NGO`s spokesman, Hammady Ould Lehbouss, told AFP.
"Twenty-three of the protesters were arrested and six were injured during clashes with police, who were using batons and large amounts of tear gas," he said.
"We wanted to organise a peaceful march. The police attacked us, and violent clashes followed," he added.
The police did not respond to the claim when contacted by AFP for comment.
The leader of the IRA is Biram Ould Dah Ould Abeid, who was a candidate for the country`s presidency in June 2014, running against incumbent Mohamed Ould Abdel Aziz.
Ould Abeid was arrested in January along with two other anti-slavery activists, one of whom was a member of the IRA, and sentenced to two years in prison for "belonging to a non-authorised organisation, protesting, and incitement to rebellion."
The third activist, Djiby Sow, is president of the cultural and civic association Kawtal Ngam Yellitaare, and was recently released on bail for health reasons.
The two jailed members of the IRA are currently appealing their sentences, which the organisation has repeatedly denounced as wrongful imprisonment. The IRA has organised a number of protests since their arrests, which have often been dispersed by authorities.