Excerpts from Books and Wikipedia
"The Zanj Rebellion (...) was a major revolt against the Abbasid Caliphate, which took place from 869 until 883. Begun near the city of Basra in present-day southern Iraq and led by one Ali ibn Muhammad, the insurrection involved enslaved Bantu-speaking people (Zanj) who had originally been captured from the coast of East Africa and transported to the Middle East, principally to drain the region's salt marshes. It grew to involve slaves and freemen, including both Africans and Arabs, from several regions of the Caliphate, and claimed tens of thousands of lives before it was finally defeated. Several Muslim historians, such as al-Tabari and al-Mas'udi, consider the Zanj revolt to be one of the "most vicious and brutal uprisings" of the many disturbances that plagued the Abbasid central government." [Wikipedia]