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, South African Communist Party (SACP) and the South African Indian Congress (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.
References

O'Malley P., Nelson Mandela's Testimony at the Treason Trial 1956-60, from the Nelson Mandela Centre of Memory, [online], Available at: www.nelsonmandela.org [Accessed: 25 April 2014]|Guiloineau J.(1939), 'Nelson Mandela: The Early Life of Rolihlahla Madiba', (North Atlantic Books), p.146