One man was killed on Tuesday when Mauritanian police fired live rounds and tear gas grenades at protesters opposing a government census effort they say discriminates against blacks.
The violence underscores deep-rooted tensions between the black and Arab populations of the West African nation, which straddles the Sahara.
"We regret that six from our group were wounded and a seventh protester, Lamine Mamgane, died from chest wounds," said Adama Ndiaye, a spokesman for the protest movement 'Don't Touch My Nationality', told Reuters by telephone.
Another member of the group confirmed the death. Two officials in the southern town of Maghama near Senegal's border where the incident took place also confirmed the information on condition of anonymity.
Tuesday's was the latest of several violent demonstrations in recent days protesting against Mauritania's new census effort, which the protesters say makes it more difficult for black people than Arabs to prove their Mauritanian nationality.
Tijane Wague, a member of the protest group in the town of Kaedi 110 km (70 miles) northwest of Maghama, said it called a truce with local authorities in return for the release of 20 of the 40 of their number arrested between Saturday and Monday.
In a statement read to state media, Interior Minister Mohamed Ould Boilil L'Etat said Mauritania was determined to see the census through and accused the protesters of acting against the national interest.
Mauritania, a poor, mostly desert nation struggling with a growing presence of al Qaeda-linked militants, has a long history of tension between its black and Arab populations.
Rights groups say hundreds of black Mauritanians were killed or went missing during inter-ethnic clashes between 1989 and 1991. The government has pledged to seek to identify the graves of the dead in a move to encourage reconciliation.