Desmond Tutu has long been among Robert Mugabe's sharpest critics
Archbishop Desmond Tutu is among activists in southern Africa who have launched a fast and hunger strike in solidarity with the people of Zimbabwe.
The new Save Zimbabwe Now movement says African leaders must abandon the policy of quiet diplomacy and recognise there is no legal government in Zimbabwe.
A Johannesburg Methodist church, long a place of refuge for Zimbabweans in exile, will be the protesters' base.
Power-sharing plans in Zimbabwe remain stalled since a deal in September.
President Robert Mugabe and opposition factions ended 12 hours of talks on Tuesday with no progress.
The activists said they would protest next Monday at a special regional summit, set for South Africa or Botswana, that has been convened in the latest effort to break the deadlock.
The BBC's Peter Biles in Johannesburg says with no end in sight to Zimbabwe's political impasse, the daily suffering of ordinary people goes on.
Food shortages, a cholera epidemic, the collapse of the health and education systems, and an economic meltdown are among Zimbabwe's litany of woes.
STATE OF ZIMBABWE
Five million people - almost half population - need food aid
Central bank introduced Z$100tr note, worth about US$30 (£20)
Unemployment more than 80%
More than 2,200 people have died in cholera outbreak
The Methodist church in the centre of Johannesburg was draped on Wednesday with anti-Mugabe banners as well as the drying laundry of the hundreds of Zimbabweans living there.
Kumi Naidoo, a veteran of the anti-apartheid movement, began a 21-day, water-only fast.
Retired Cape Town Archbishop Tutu, 77, long among Mr Mugabe's sharpest critics, will fast one day a week.
The Nobel laureate has in the past urged the international community to use the threat of force to oust Mr Mugabe.
Human rights activist Graca Machel, who is also the wife of former South African President Nelson Mandela, attended the launch at the church but said she would not be fasting for personal reasons.
The 63-year-old said of Mr Mugabe's administration: "Any government which goes and assaults its own people has lost any kind of legitimacy, completely."
Wilson Mugabe, a pastor from Zimbabwe who is not related to its president, broke down in tears as he told the congregation about the plight of his country men and women.
"Please hear us as Zimbabweans, we've suffered enough," he said.
Ms Machel and Mr Tutu are both members of the Elders, an international team of statesmen founded by Mr Mandela and including ex-US President Jimmy Carter, which tries to find solutions to global crises.
In November, Mr Mugabe's government barred the Elders from making a fact-finding tour of Zimbabwe.