The long-banned ANC has been in power ever since it achieved its aim of ending white minority rule in 1994 with the end of the apartheid system of racial segregation.
It is celebrating the milestone with a gathering of dignitaries from across the continent. The Prime Minister sent congratulations in a letter to President Jacob Zuma.
“On behalf of the British people, I want to congratulate you and everyone involved with the African National Congress on this very special anniversary,” he wrote.
“The African National Congress has been a beacon for the world in the fight against discrimination and the struggle for freedom from oppression.
“It stood up for the rights of all South Africans. And the names of great men like Albert Luthuli, Oliver Tambo and Nelson Mandela will forever be an inspiration not just to the people of South Africa, but to people all over the world.”
Cameron said the changes achieved since the end of apartheid had been “incredible” but pledged UK support in dealing with the challenges of the next 100 years. “We will stand with you in the ongoing struggle for equality, democracy and prosperity for the people of South Africa, and for justice and freedom from tyranny around the world.”
Among the challenges it faces are a jobless rate of about 36 per cent to 70 per cent among young people – and complaints that it has failed to improve economic inequalities and hardship.
The ANC, which has also faced a string of corruption scandals and internal disputes, was set up by a group of black South Africans in a small church in Bloemfontein in January 1912.
Yesterday’s celebrations included an address by President Zuma and a gala dinner. One notable absentee was former president Nelson Mandela who, at the age of 93, rarely appears in public.