JOHANNESBURG — A former anti-apartheid activist and medical doctor who also held a senior position at the World Bank will be the presidential candidate of South Africa's main opposition party in this year's elections.

Mamphela Ramphele had a son with activist Steve Biko, who in 1977 was tortured and died in police custody. Last year, Ramphele formed her own party to challenge the African National Congress, which has been the ruling party since the first all-race vote in 1994.

Ramphele said Tuesday that her decision to merge with the Democratic Alliance, a larger opposition group, is in the country's best interests.

President Jacob Zuma and the ANC are the electoral front-runners but they have lost some support because of corruption, poverty, unemployment and a lack of adequate government services.