The Government of the Republic of South Africa announced in Parliament that 18,000 people had been arrested and detained since the declaration of the State of Emergency in March. The State of Emergency was proclaimed on 30 March 1960, following the Sharpeville killings that left 69 unarmed people dead and scores injured. Following the proclamation, 11 727 people were detained. In April 1960, the government banned  the African National Congress (ANC), the Pan Africanist Congress (PAC) and other organisations under the Unlawful Organisations Act. These organisations were left with no choice, but to operate underground.
References

O’Malley, P. ‘1960’ from Nelson Mandela Center of Memory, [online], available at www.nelsonmandela.org.za (Accessed: 12 April 2013)|

South African History Online, ‘State of Emergency follows Sharpeville massacre’, [online], available at www.sahistory.org.za (Accessed: 12 April 2013)|

South Africa. Info, ‘Sharpeville remembered’, [online], available at www.southafrica.info (Accessed: 12 April 2013)