* Ramphele to make political announcement next week
* Support seen among middle-class professionals
* ANC says ready, able to fend off challenge (Adds quotes, background on Ramphele, details on politics)
By Jon Herskovitz
JOHANNESBURG, Feb 13 (Reuters) - Respected anti-apartheid activist Mamphela Ramphele resigned as chairwoman of Gold Fields on Wednesday, intensifying speculation she is about to launch a political party to challenge the ruling African National Congress (ANC).
The medical doctor and former World Bank managing director will make a statement about her "political plans" on Johannesburg's Constitution Hill on Monday, a public relations consultancy working for her said this week.
Gold Fields, one of South Africa's biggest mining companies, said Ramphele had decided to retire "to further her socio-economic and political work".
The 65-year-old commands considerable respect among South Africa's black majority as a partner of Black Consciousness leader Steve Biko, who died in 1977 as a result of beatings received in an apartheid prison.
She was also placed under house arrest for seven years by the apartheid government because of her political work. Even after the advent of democracy in 1994, she has seldom shied away from challenging authority and questioning the ANC's leadership of the self-styled "Rainbow Nation".
At a business breakfast this week she described the ANC as "authoritarian, intolerant of criticism and unaccountable", according to the Daily Maverick newspaper, and in 2012 accused President Jacob Zuma of leading an assault on the post-apartheid constitution.
"There is enough concern about the utterances by ANC leaders and government officials to suggest that not all is well in our constitutional democracy," she told a meeting in the industrial city of East London.
Ramphele, also a noted author, public health activist and academic, is entering the political arena without a grass-roots network, meaning her party may not garner significant support in presidential elections next year.
But she would not be taking on the might of Nelson Mandela's 101-year-old liberation movement without having a solid plan in place, political analyst Nic Borain said.
"She is an amazingly powerful woman," he said. "She would have done her homework."
The ANC is expected to storm to victory in the 2014 polls, but the party has seen its support wane over the years as corruption has eaten into welfare programmes aimed at helping the nearly 40 percent of the population mired in poverty.
Election results show those dissatisfied with the ANC have mostly stayed away from the polls instead of turning out in large numbers for the main opposition Democratic Alliance, seen as representing white privilege.
Zuma, a 70-year-old Zulu traditionalist who has called for young ANC members to obey their elders, has looked to older voters in rural areas for support instead of the young blacks flocking to the cities.
Last year he told parliament he was worried about black people "who become too clever" because they could become the sharpest critics of African tradition and culture.
His right-hand-man took a pre-emptive shot at Ramphele last month, saying her party would be a voice in the wilderness.
"It remains to be seen if what she created - or will create - changes anything. We will contest elections against anyone and are confident we will succeed," Gwede Mantashe, the ANC secretary-general, told the Mail & Guardian newspaper.
The ANC, with a massive organisation that commands respect for ending apartheid, has already fended off a major political challenge when a group of party heavyweights broke off to form the Congress of the People (COPE) in 2008.
Seen as a political game changer at the time, COPE is now a marginal player riven by in-fighting and starved of cash.
Ramphele, whose parents were teachers, grew up in a rural part of the country and was one of the few blacks who earned a medical degree in the apartheid state.
She has criticised the ANC for crushing the dreams of the liberation and letting cronyism undermine the ideals of "the struggle".
"We need to shift the frame of reference from the politics of fear and patronage, to assert ourselves as sovereigns and defend our constitutional democracy," she wrote in her 2012 book "Conversations with My Sons and Daughters." (Additional reporting by Sherilee Lakmidas; Editing by Jon Hemming)