"We have Mahatma Gandhi at the back of our minds as we embark on this hunger strike," Save Zimbabwe Now said.
"Fasting has been chosen to symbolise the hunger and discomfort faced by millions of Zimbabweans every day in varying forms of severity."
The president of the World Alliance for Citizen Participation, Kumi Naidoo, and chairman of the SA Gender Commission, Nombuso Gasa, will go without food, but with water, for 21 consecutive days.
They will stay at the Central Methodist Mission Church in Johannesburg during the fast.
Tutu will be among those taking part in the fast every Wednesday for three months.
In Harare, southern African leaders planned a new summit to break Zimbabwe’s political impasse, a day after Robert Mugabe and rival Morgan Tsvangirai ended marathon talks without a deal.
After 12 hours of talks on Monday, Mugabe said he had accepted a proposal from the Southern African Development Community that would have seen Tsvangirai sworn in as prime minister on Saturday.
However, a bitter and angry Tsvangirai left the talks – mediated by President Kgalema Motlanthe – refusing to accept the post until crucial issues were resolved.
"Unfortunately, there’s been no progress because the same outstanding issues on the agenda are the issues that are creating this impasse," Tsvangirai said. – Sapa-AP-AFP