Democratic Alliance leader Helen Zille on Friday accused the ANC of subverting the judiciary and state institutions to settle political scores within the ruling party.

"In the six months since April, we have had ANC party control taking over the independent institutions of the state," Zille wrote in her weekly newsletter.

She said prime examples were the "political abuse" of the National Prosecuting Authority (NPA) to abandon the corruption charges against President Jacob Zuma and that of the Judicial Service Commission (JSC) to exonerate Western Cape Judge President John Hlophe because "he is a champion of the Zuma faction".

"The NPA put loyalty to its political master before the rule of law when it dropped charges against Zuma," she said.

The DA reacted with outrage this week when, in response to its application for a judicial review of that decision, Zuma's lawyer Michael Hulley filed an affidavit arguing that the sitting head of state could not be charged with criminal conduct.

He added it was "simply not legally feasible to turn back the clock".

Zille dismissed the submission as "disturbing nonsense".

Zuma on Thursday declined to comment in detail on the affidavit, but said he did not consider himself to be above the law and would testify if called before court.

Said Zille: "My great fear is that under the cloak of Zuma's charm, his cadres are subverting the South African Constitution for political ends and trying to corrupt the independent institutions of the state, especially the judiciary, into tools of the ANC."

She said Zuma was obviously mindful that powerful institutions could be turned against him if, like former president Thabo Mbeki, he lost the support of powerful factions in the ANC.

"Since the ANC cadres are always keenly aware of power play within the party, if the play turned against Zuma, the NPA could quickly lay criminal charges against him, a suitable judge could try and convict him, and he could be thrown out of the presidency and perhaps into goal.

"We are sinking into a kind of gangsterism when the decisions of the state justice system depend not on the law but on party intrigue.

"That's why the faction fights inside the ANC are so vicious."

Zille said the DA believed that independence of the judiciary was the bedrock of decent societies and should be sacrosanct.

"The assault on this independence by the ANC is of the gravest concern."