Thirteen anti-slavery campaigners were sentenced for up to 15 years in prison in Mauritania last week, for their role in a protest aimed at denouncing the practice of slavery in the country. The government tribunal found members of the Initiative for the Resurgence of the Abolitionist Movement (IRA) guilty of various counts, including attacks against the government, armed assembly and membership of an unrecognized organization.

Mauritania is the world’s last country to abolish slavery, and the country didn’t make slavery a crime until 2007. The practice reportedly affects up to 20% of the country’s 3.5 million population (pdf, p. 258), most of them from the Haratin ethnic group.

For centuries, the black Haratins have been caught in a cycle of servitude enforced by the white Moors, who are lighter-skinned descendants of Arab Berbers. Some members of the Haratin group are sometimes born into slavery, and their masters are able to sell them or buy them as gifts. They mostly have no rights, receive little education or pay, and may not inherit property or give testimony in court. There’s also been reports of government collusion with Arab Berbers into intimidating slaves who break free from their masters.

Organizations like the IRA say the anti-slavery laws are not enough, and have consistently labeled Mauritania as slavery’s last stronghold. The Mauritanian government has consistently refused to register the organization as a non-governmental entity, according to Freedom House.

The IRA was founded in 2008 by Biram Dah Abeid, himself a descendant of slaves. Abeid was released in May this year after spending 18 months in prison, with the Supreme Court overturning a two-year sentence against him and his assistant Brahim Bilal for protesting without authorization.