The Nelson Mandela Foundation has sharply criticised South Africa’s president, Jacob Zuma, saying that “political meddling for private interests” during his tenure has weakened state institutions and poses a threat to the country’s democracy.

“It is painful for us at the Nelson Mandela Foundation to bear witness to the wheels coming off the vehicle of our state,” the organisation said on Tuesday.

The intervention is unusual because the foundation, created by Mandela, has traditionally avoided making political statements. Instead, it has focused on themes such as justice and equality, as well as the legacy of racism dating from South Africa’s era of white minority rule, which ended in 1994 when Mandela became the country’s first black president.

The statement adds to the pressure on Zuma, who has lost popularity even within his own ruling African National Congress party after a series of scandals.

One of those unfolded in court on Tuesday as government lawyers argued against the release of a report into claims that a business family linked to Zuma sought to influence cabinet postings.

“The public has got a right to this report as soon as possible,” said Dali Mpofu, an opposition lawyer.

Opposition groups have planned a demonstration in Pretoria, South Africa’s administrative capital, on Wednesday. The rally was intended to coincide with a court appearance by the finance minister, Pravin Gordhan, who faced a fraud charge that his supporters said was a politically motivated move by pro-Zuma factions.

Organisers decided to go ahead with the protest even after the charges against Gordhan were dropped.

“We have seen a weakening of critical institutions such as the South African Revenue Service, the National Prosecuting Authority and law enforcement bodies due to political meddling for private interests,” said Njabulo Ndebele, the chairman of the Nelson Mandela Foundation.

Ndebele also referred to the scandal surrounding Zuma’s use of more than $20m (£16m) in state funds to upgrade his rural home, a process that was ruled unconstitutional in March.

Zuma apologised and paid back more than $500,000, as determined by the treasury. “It is increasingly a national consensus that he has failed the test [of his constitutional obligations],” Ndebele said.

The foundation chairman did not directly call for Zuma’s resignation, as some South Africans have done, but urged the ruling party to ensure that the state is put “in safe and capable hands”.

Bongani Ngqulunga, a spokesman for Zuma, said he had not seen the statement and could not immediately comment.

Zuma retains powerful supporters in the government despite growing rifts within the ruling party, which suffered heavy losses in local elections earlier this year, partly because of public dissatisfaction with the president.