"The decision to establish this party is a decision that is also accompanied by a determination that this must be a South African initiative, supported by South Africans," she told SABC Morning Live in an interview on Tuesday.
She said the financial support behind the party, which was launched in Johannesburg on Monday, was not from Americans, as had been insinuated.
Earlier on Tuesday, ANC secretary general Gwede Mantashe reportedly said the new political platform could possibly be used to destabilise the country.
"The worry that we are raising ... is that when these initiatives were announced, the founder was in the United States and announced that she is raising funding," Mantashe said.
Last year, Ramphele visited the US to speak at Brown University.
She said that on her way back, she had met South Africans at a friends' cocktail party – as she had in other parts of the world.
"I went to Australia last year and briefed South Africans living in the diaspora ... They are as much South African as we are," she said.
2014 national elections
"The money we are currently using comes from South Africans, right here at home."
Ramphele said she intended using Agang SA to contest the 2014 national elections.
After Ramphele's speech at Constitution Hill in Johannesburg, the ANC said in a statement issued on Monday night, together with a release on a national working committee visit to Limpopo that it hoped "the pumping of foreign funds in South Africa will not undermine the further democratisation and transformation in our country".
The ruling party also slated the "undefined party political platform" for not clarifying its intentions.
"It is also a disservice to the people of South Africa for those who are behind such formation not to have given clarity of what it is, that is new that they want to bring to the political terrain."
The Democratic Alliance (DA) merely "noted" the announcement and expressed a willingness to engage with her in the coming months.
"Dr Ramphele shares the DA's core values of non-racialism and constitutionalism, and her move is another step in the long process of realigning South African politics around these values," it said. – Sapa