The Nelson Mandela Foundation (NMF) has called for South Africa's President Jacob Zuma to be sacked.
He had "failed the test" of leadership and South Africa needed to hold to account those who were "looting" state resources, it said.
Mr Zuma has been dogged by allegations of corruption for more than a decade.
The call by the NMF, which is run by colleagues of South Africa's first black president, is the latest sign of growing discontent with his rule.
Its intervention came as a court began hearing a bid by Mr Zuma's legal team to prevent the release of a report by former anti-corruption chief Thuli Madonsela.
She investigated allegations that he let the wealthy Gupta family wield undue influence in his government. Both Mr Zuma and the Guptas have denied any wrongdoing.
In a statement entitled Time to account for crippling the state, the NMF said the governing African National Congress (ANC) needed to take steps to put the government back in "safe and capable hands".
"Twenty years since Nelson Mandela signed South Africa's constitution into law and as the third anniversary of his passing approaches, it is painful for us at the Nelson Mandela Foundation to bear witness to the wheels coming off the vehicle of our state," the NMF added.
Mr Mandela spent more than 27 years in prison for fighting white-minority rule and became South Africa's first democratically elected leader in 1994. He stepped down five years later and died in December 2013 at the age of 95.
The NMF said South Africa's democracy was now under a "real threat", with key government institutions being used to advance "private interests".
"We are reaping the results of a political trend of personalising matters of state around a single individual leader. This in a constitutional democracy is to be deplored," it added.
Ms Madonsela's investigation was triggered by allegations in March by Deputy Finance Minister Mcebisi Jonas that the Gupta family had made "a mockery of our hard-earned democracy" by offering him the finance minister's post last year.
Mr Jonas said he rejected the offer; the Guptas accused him of political point-scoring.
Former ANC MP Vytjie Mentor also alleged that the family offered her the powerful public enterprise minister's post in 2010 in exchange for business favours.
Ms Mentor alleged that Mr Zuma was in another part of the Gupta's family home in Johannesburg when the offer was made. Mr Zuma's office said at the time that he had no "recollection" of Ms Mentor, while the family strongly denied her allegation.
Ms Mentor and opposition parties, including the Democratic Alliance (DA) and Economic Freedom Fighters (EFF), are challenging Mr Zuma in court, insisting that the report be released.
EFF supporters are protesting outside the High Court in the capital, Pretoria, to demand its publication.
Mr Zuma's court action prevented Ms Madonsela from publishing the report before she stepped down as public protector at the end of her term last month.
He said it would be unlawful to release the report as he had not been given enough time to respond to the allegations.
This was the second time Ms Madonsela investigated Mr Zuma during her seven-year term.
In March, South Africa's highest court upheld her findings that Mr Zuma "unduly benefited" from government money used to upgrade his private rural home.
It led to widespread calls for Mr Zuma to resign, but he survived a DA-sponsored impeachment vote in parliament after ANC MPs rallied behind him.
Correspondents say while the contents of Ms Madonsela's latest report are unclear, Mr Zuma will almost certainly face another impeachment vote if there are any adverse findings about him.
The president is also at the centre of another case and is trying to overturn a unanimous ruling of a High Court that he should stand trial on 783 counts of corruption in relation to an arms deal negotiated in 1999.