Cape Town - Agang SA leader Mamphela Ramphele has described her decision to be the DA's presidential candidate as an "astonishing moment" in South Africa's history.
"I believe that this decision is in the best interest of South Africa as we head into turbulent waters," she told reporters in Cape Town on Tuesday.
"Today is another astonishing moment in what we offer to the people of South Africa, and once again, to the world...
"We are the government in waiting," said Ramphele, flanked by Democratic Alliance leader Helen Zille.
Earlier, Zille said: "I can announce today that Dr Mamphela Ramphele has accepted the DA's invitation to stand as our presidential candidate in the 2014 general election."
"This is a game changing moment for South Africa," said Zille.
"I know Mamphela as a principled, fiercely determined person who loves our country very deeply... I can think of no better person to be our presidential candidate in this crucial, tipping point election," said Zille.
Even ANC supporters wanted political change, Ramphele said.
"Good people in the ANC have reached out to us, but are scared," she told reporters.
"Good business leaders have reached out to us but are nervous."
Ramphele said millions of people wanted to make a different choice in this year's election.
"This is your government-in-waiting.
"I will not lead you, not lead us, nor lead South Africa on the path of conflict but of commitment - to you, to your children, to health, to high quality education and job opportunities," she said flanked by Zille.
- Watch a clip of Mamphela Ramphele, filmed earlier this month, discussing Agang’s finances.