In early 1964, a new crisis broke out as Congolese rebels calling themselves "Simba" (Swahili for "Lion") rebelled against the government. They were led by Pierre Mulele, Gaston Soumialot and Christophe Gbenye who were former members of Gizenga's Parti Solidaire Africain (PSA). Mulele was an avowed Maoist, and for this reason his insurgency was supported by communist China.

The rebellion affected Kivu and Eastern (Orientale) provinces. By August they had captured Stanleyville and set up a rebel government there. As the rebel movement spread, discipline became more difficult to maintain, and acts of violence and terror increased. Thousands of Congolese were executed, including government officials, political leaders of opposition parties, provincial and local police, school teachers, and others believed to have been Westernized. Many of the executions were carried out with extreme cruelty, in front of a monument to Lumumba in Stanleyville.