The majority ruling by South Africa's top court, sentencing Jacob Zuma to 15 months in jail, was an "emotional and angry" unconstitutional decision, the former president's foundation said on Wednesday.

Zuma failed to appear at a corruption inquiry led by Deputy Chief Justice Raymond Zondo in February, prompting the inquiry's lawyers to ask the Constitutional Court to seek an order for his imprisonment.

On Tuesday, the court sentenced Zuma to jail, and gave him five days to appear before police.

The Jacob Zuma Foundation said in a statement that Zuma had never believed he was above the law, but had only wanted his rights protected.

"Our Patron has expressed his doubts about the lawfulness of the Zondo Commission, the biased manner in which it is being conducted, and the fact that it has been transformed into a "slaughterhouse" and a forum in which all kinds of unsubstantiated and defamatory allegations have been made against him," the foundation said.

Zondo has denied being biased against Zuma and has dismissed Zuma's bid to recuse him as chairperson of the inquiry.

The Zondo inquiry is examining allegations of high-level graft involving the Gupta brothers, three Indian-born businessmen, during Zuma's time in office from 2009 to 2018. Zuma denies wrongdoing and has so far not cooperated. The Guptas have also denied wrongdoing.

Justice Sisi Khampepe, who read Tuesday's judgment, refuted a 21-page letter that Zuma sent to the country's chief justice in which he claimed to have been treated unfairly.

"His attempts to evoke public sympathy through unfounded allegations fly in the face of reason, and are an insult the constitutional dispensation for which so many women and men fought and lost their lives," Khampepe said.

But Zuma's foundation said the ruling was contrary to the rule of law, arguing the commission was given an advantage in a case that was adjudicated by Zondo's colleagues.

"The Jacob Zuma Foundation denounces Judge Khampepe's judgment as judicially emotional and angry and not consistent with our constitution."

"At a bare minimum. ..the courts must act independently and without bias, with unremitting fidelity to the law, and must be seen doing so. That did not happen in the constitutional court," the statement read.