"(The ANC) must stop putting the interests of Jacob Zuma before the interests of South Africa," it said.
South African President Jacob Zuma on Thursday faced increasing calls to resign after a damaging probe unearthed further evidence of alleged corruption at the top of the ANC government.
The country's anti-graft watchdog ordered a judicial inquiry into possible criminal activity in Zuma's relationship with the Guptas, a wealthy Indian family accused of wielding undue political influence.
Zuma, 74, has weathered a series of major scandals since coming to power in 2009, but rapidly declining support for the African National Congress (ANC) party has threatened his presidency.
Fears have grown that South Africa has thrown away the optimistic legacy of Nelson Mandela's post-apartheid era -- with Zuma's alleged misconduct at the heart of public discontent.
"The mood of the people has become more and more angry," said a statement from Save South Africa, a new umbrella group of anti-apartheid veterans, civil activists, business leaders and churches.
"(The ANC) must stop putting the interests of Jacob Zuma before the interests of South Africa," it said. "The president must step down now."
Criticism of Zuma has spread in recent months, with calls for him to step down emerging from influential trade unions, corporate bosses and even senior ANC figures, as well as from opposition parties.
"The individuals who are now joining the chorus are not only the usual suspects, but, until very recently, staunch backers of Zuma's ANC," the Times daily said in its editorial on Thursday.
"Voices expressing dissatisfaction with his leadership are getting louder as more damning evidence of his misgovernance emerges."
The probe released by the Public Protector watchdog detailed allegations that Zuma ensured the Gupta family won huge preferential contracts with state companies and were able to choose cabinet ministers.
Deputy Finance Minister Mcebisi Jonas told the investigation he was taken by Zuma to the Guptas' home in Johannesburg where Ajay Gupta said he would be appointed finance minister.
When Jonas refused, Ajay allegedly offered him 600 million rand ($44 million) and asked if Jonas had a bag to take away 600,000 rand in cash immediately.
The report also unveiled phone evidence that David Van Rooyen, then a little-known Zuma loyalist, was regularly in touch with the Guptas before he was briefly appointed finance minister last year.
The Guptas have denied all wrongdoing, and on Thursday welcomed an official inquiry.
"Now we have opportunity to... present our facts to a judge," family lawyer Gert van der Merwe told the eNCA news channel.
Zuma, who travelled to Zimbabwe on Thursday, said that he was studying the Public Protector's report and considering a court challenge.
The president, who under apartheid was jailed on Robben Island with Mandela, has been dogged by scandals, including being found guilty of violating the constitution after he refused to repay taxpayers money used to refurbish his private rural house.
But he retains deep loyalty in large sections of the ANC, especially in rural areas, and has built up a strong patronage network of supporters.
The ANC is due to choose a new leader at the end of next year, with the selected leader then running as president in national elections in 2019, when Zuma cannot stand for a third term in office.
Municipal elections in August produced the ANC's worst-ever poll performance, though it remains easily South Africa's biggest party.
Public frustration has been fanned by the government's failure to tackle soaring unemployment, low growth and limited change for many of the poorest since the end of white-minority rule in 1994.