Mauritania has sentenced 13 anti-slavery activists to prison on terms ranging from three to 15 years.

Amnesty International said late Thursday that the sentences send a disturbing message that Mauritanian authorities are willing to silence human rights defenders and restrict freedom of expression. It called again for the activists' unconditional release.

The activists are members of a non-governmental organization that campaigns against slavery in the West African country. They were found guilty Thursday of rebellion, armed assembly, use of violence, attack against public authority and membership in an unrecognized organization.

They were arrested between the end of June and early July after a protest against the relocation of a slum community.

Mauritania outlawed slavery in 2007, but the practice persists and slavery convictions remain rare.