Exports of slaves to the Muslim world via the Indian Ocean became more defined after Muslim Arab and Swahili traders won control of the Swahili Coast and sea routes during the 9th century. These traders captured Bantu peoples (Zanj) from the interior in the present-day lands of Kenya, Areas around lake Victoria, Mozambique and Tanzania and brought them to the coast. Muslim merchants traded an estimated 1000 East African slaves annually between 800 and 1700, a number that grew to c. 4000 during the 18th century, and 3700 during the period 1800–1870.
When estimating the number of people enslaved from East Africa, author N’Diaye and French historian Olivier Pétré-Grenouilleau estimate 17 million as the total number of people transported from the 7th century until 1920, amounting to an average of 6,000 people per year. Many of these slaves were transported by the Indian Ocean and Red Sea via Zanzibar. The captives were sold throughout the Middle East. This trade accelerated as superior ships led to more trade and greater demand for labour on plantations in the region. Eventually, tens of thousands of captives were being taken every year.
Slave labor in East Africa was drawn from the Zanj, Bantu peoples that lived along the East African coast mainly as well as, outcasts and victims of tribal wars from within the heart of Africa including Uganda. The Zanj were for centuries shipped as slaves by Muslim traders to all the countries bordering the Indian Ocean. The Umayyad and Abbasid caliphs recruited many Zanj slaves as soldiers and, as early as 696, there were revolts of Zanj slave soldiers in Iraq. A 7th-century Chinese text mentions ambassadors from Java presenting the Chinese emperor with two Seng Chi (Zanj) slaves as gifts in 614.