David Cameron hailed the African National Congress (ANC) as a "beacon for the world" in fighting discrimination and oppression as the South African party celebrated its centenary.
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 huge gathering of dignitaries from across the continent and the Prime Minister sent the UK's 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 an unemployment rate of around 36% - 70% among young people - and complaints it has failed sufficiently to improve economic inequalities and hardship.
The ANC has also faced a string of corruption scandals and internal disputes.