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.