A Mauritanian official says that police have arrested 56 people following violent clashes over a population census.
Ministry spokesman Dahmane Ould Beyrouk said Friday that the demonstrators had burned cars during an unauthorized march.
The protesters are upset about a government census, fearing it will discriminate against black Mauritanians and call their citizenship into question.
Black Mauritanians have long faced repression in this West African country. Although slavery was banned in 1981, it was not established as a crime punishable by prison time until 2007.
Authorities in Mauritania say they want to conduct the census in order to improve national security. The country has faced a growing number of attacks from al-Qaida's North Africa branch.