Nelson Mandela, leader of the Transvaal branch of the African National Congress (ANC), was acquitted of treason charges after a four year long trial. Mandela was one of the 156 leading figures, 104 Africans, 23 Whites, 21 Indians and 8 Coloureds, of the South African Congress of Democrats (SACOD), who were arrested on the morning of 5 December 1956, but all eventually acquitted. The SACOD constituted mainly of three organisations, namely the ANC, SACP and SAIC.  These mass arrests were executed a year after the Congress of People met at Kliptown in 1955 to endorse the Freedom Charter and was an attack on the Charter.

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Sources:

South African History Online http://home.att.net/~g.michon/march29/ http://www.anc.org.za/ancdocs/history/trials/