22 March 1961
Pan-Africanist and first president of Ghana, Kwame Nkrumah, was a leader who strived for a united Africa. Nkrumah was one of the founders of the Organisation of African Unity (OAU) and its first president. On 22 March 1961, Nkrumah called on world leaders to impose sanctions on South Africa because of its Apartheid policies. African harbours were closed to the South African government, and South African aircrafts were prohibited from flying over the rest of the continent. In addition, the OAU managed to convince the United Nations (UN) to expel South Africa from bodies like the World Health Organisation (WHO).
References

SAHO, Organisation of African Unity (OAU), from South African History Online, [online], Available at www.sahistory.org.za [Accessed: 06 March 2014]|Ojokwu, H., Kwame Nkrumah: A true champion of African unity from Uhuru Spirit, from Uhuru Spirit, [online], Available at https://uhuruspirit.org [Accessed: 06 March 2014]