A court in South Africa is hearing a case brought by the opposition to reinstate 738 corruption charges against President Jacob Zuma.
His office said he would oppose the bid, as prosecutors acted “rationally” when dropping the charges in 2009.
The opposition believes the decision was political, and opened the way for Mr Zuma to become president.
He was accused of taking bribes over a multi-billion dollar arms deal, but strongly denied the allegation.
At the time, the National Prosecuting Authority (NPA) said phone-tap evidence, dubbed in the local media as “spy tapes”, suggested political interference in the investigation, and it was “unconscionable” to press ahead with the case.
The main opposition Democratic Alliance (DA) party is challenging the decision in the High Court after a nearly six-year legal battle.
It said in court papers that the NPA’s decision was unconstitutional and irrational.
The DA has also tabled a no-confidence motion against Mr Zuma in the National Assembly, the lower house of parliament.
South Africa was “spiralling downward, and doing so at an alarmingly fast rate” under his presidency, it said in a statement.
The governing African National Congress (ANC) party, which commands an overwhelming majority, is expected to defeat the motion.
Despite the “frivolous antics” of the DA, its confidence in Mr Zuma remained unshaken, the party said.
Mr Zuma, a former ANC intelligence chief, has been dogged by controversy since he became president in May 2009 after the corruption charges were dropped the month before.
Last month, he conceded in court that he needed to pay the government for money used to upgrade his private home, including the building of a swimming pool, amphitheatre, chicken run and cattle enclosure.
That case was brought by the DA and left-wing Economic Freedom Front (EFF) party after Mr Zuma failed to pay the money, despite a ruling against him by an independent anti-corruption body in 2014.
In December, South Africa’s currency went into a tail spin after having three finance ministers in a week.