by two insurgent groups, the People's Movement for the Liberation of Angola (MPLA), and the União das Populações de Angola (UPA), which became the National Liberation Front of Angola (FNLA) in 1962. The MPLA commenced activities in an area of Angola known as the Zona Sublevada do Norte (ZSN or the Rebel Zone of the North), consisting of the provinces of Zaire, Uíge and Cuanza Norte. Insurgent attacks On February 4, 1961, using arms largely captured from Portuguese soldiers and police 250 MPLA guerrillas attacked the São Paulo fortress prison and police headquarters in Luanda in an attempt to free what it termed 'political prisoners'. The attack was unsuccessful, and no prisoners were released, but seven Portuguese policemen and forty Angolans were killed, mostly MPLA insurgents. Portuguese authorities responded with a sweeping counterinsurgency response in which over 5,000 Angolans were arrested, and a Portuguese mob raided the musseques (shanty towns) of Luanda, killing several dozen Angolans in the process. On March 15, 1961, the UPA led by Holden Roberto launched an incursion into the Bakongo region of northern Angola with 4,000–5,000 insurgents. The insurgents called for local Bantu farmworkers and villagers to join them, unleashing an orgy of violence and destruction. The insurgents attacked farms, government outposts, and trading centers, killing everyone they encountered, including women, children and newborns. In surprise attacks, drunken and buoyed by belief in tribal spells that they believed made them immune to bullets, the attackers spread terror and destruction in the whole area. At least 1,000 Portuguese settlers and an unknown but larger number of indigenous Angolans were killed by the insurgents during the attacks. The violence of the uprising received worldwide press attention and engendered sympathy for the Portuguese, while adversely affecting the international reputation of Roberto and the UPA. Portuguese response In response, Portuguese Armed Forces instituted a harsh policy of reciprocity by torturing and massacring rebels and protesters. Some Portuguese soldiers decapitated rebels and impaled their heads on stakes, pursuing a policy of "an eye for an eye, a tooth for a tooth". Much of the initial offensive operations against Angolan UPA and MPLA insurgents was undertaken by four companies of Caçadores Especiais (Special Hunter) troops skilled in light infantry and antiguerrilla tactics, and who were already stationed in Angola at the outbreak of fighting. Individual Portuguese counterinsurgency commanders such as Second Lieutenant Fernando Robles of the 6ª Companhia de Caçadores Especiais