A tribunal in Mauritania has sentenced 13 anti-slavery activists to up to 15 years in prison for their role in a riot in June in a decision condemned on Friday by international campaigners as a 'devastating blow'.

The West African nation is a focus of activism by the modern anti-slavery movement over a practice believed to affect between four and 20 percent of the population.

Authorities arrested the 13 members of the Resurgence of the Abolitionist Movement (IRA) in late June and early July after a protest, many of whom are themselves former slaves.