A South African commission hearing testimonies about rampant state corruption during Jacob Zuma's reign said Monday that it would ask police to investigate the former president after he walked out last week.

After months of playing cat-and-mouse, Zuma appeared before the panel last week seeking what he called an "impartial" judge, demanding that the commission's chair, deputy chief justice Raymond Zondo, recuse himself.

But Zondo tossed out this application on Thursday, saying it "failed to meet the test for a reasonable apprehension of bias."

Shortly after the ruling, the commission took a few minutes recess, but Zuma and his lawyer Muzi Sikhakhane did not return.

Zuma had been expected to face a request to take to the witness stand.

On Monday, the commission announced it would "lay a criminal complaint with the South African police against Mr Zuma so that the police can investigate his conduct".

The commission will now issue a fresh summons to force Zuma to return to testify.

Judge Zondo said Zuma's behaviour risks sending a wrong message to the rest of the witnesses that if they were uncomfortable to answer questions they could simply excuse themselves and "can come and go as they please".

Former South African president Jacob Zuma walked out of a commission hearing testimonies over mass corruption during his rule
Former South African president Jacob Zuma walked out of a commission hearing testimonies over mass corruption during his rule AFP / GUILLEM SARTORIO

"If that were to happen this commission would not be able to operate," Zondo said.

"It is therefore quite important for the proper functioning of this commission that Mr Zuma's conduct be dealt with in a manner in which our law provides it should be dealt this."

The commission, which has been hearing testimonies since 2018, said it will also urgently approach the country's top tribunal, the Constitutional Court, seeking an order that will compel Zuma "to comply with the summons".

Zuma is suspected of having enabled the widespread looting of state assets during his 2009-18 presidency.

Lavish government contracts were awarded to an Indian business family, the Guptas, among other scandals.

The 78-year-old has only testified to the commission once, in July 2019, but pulled out after a few days, saying he was being treated as an "accused" rather than as a witness.

At least 34 witnesses have directly or indirectly implicated Zuma, who has denied any wrongdoing.

In the face of the corruption scandals, the ruling African National Congress (ANC) in February 2018 forced Zuma to step down.

He was succeeded by Cyril Ramaphosa, who has vowed to confront the "scourge" of corruption.