A Mauritanian appeals court overturned three convictions and reduced the sentences of 10 other anti-slavery activists Friday for their alleged role in a riot in June.

The 13 activists are members of the Initiative for the Resurgence of the Abolitionist Movement (IRA). They were sentenced in August to up to 15 years in prison after a protest against eviction by residents of a slum in the Mauritanian capital, Nouakchott. Many of the protesters were former slaves. The 13 activists were initially convicted of using violence, attacking security, gathering while armed and being a part of an unrecognized organization - namely, the IRA.

Amnesty International's West Africa campaigner Kine Fatim Diop said the activists were "unfairly sentenced" for "peacefully expressing their opinions." Three activists remain behind bars. Diop said their continued imprisonment was a "distressing sign of the shrinking space" afforded to human rights campaigners in Mauritania.

UN experts said the trial was marred by rights violations, including torture in detention and irregularities during court proceedings.

Slavery remains in Mauritania

Despite slavery officially being outlawed in Mauritania in 1981, the West African nation remains a hotbed of slavery. Modern-day slavery in Mauritania exists under a caste system of servitude that forces those in the "slave" caste to work as cattle herders or domestic servants without pay.

Mauritania has tried to crack down on slavery and passed a law in 2015 that made slavery a crime against humanity. But campaigners say this is not enough to eradicate the practice in the West African nation.