Mr Zuma was acquitted of rape in a separate case in 2006
South African cartoonist Jonathan Shapiro at the centre of a row over his drawing of ruling party leader Jacob Zuma has admitted it is "derogatory".
But he told the BBC it was defendable, after reports that he may face legal action over the cartoon showing Mr Zuma about to rape the "justice system".
Protests are planned outside the court which is to decide if a corruption case against Mr Zuma can proceed on Friday.
Mr Zuma, who denies the graft charges, defended his right to criticise judges.
The African National Congress (ANC) leader, who is favourite to become president next year, made the comments in a lecture to students on Tuesday, two days after the Sunday Times newspaper's cartoon appeared.
He said he did not believe judges "should be beyond criticism", as long as it was informed and did not undermine their independence.
"It is only in dictatorships and autocracies where criticism is viewed with contempt," the Associated Press agency quoted him as saying.
Meanwhile, former South African President FW de Klerk has written to current President Thabo Mbeki, calling on him to actively protect the judiciary against intimidation from supporters of Mr Zuma.
Replying to the letter, President Mbeki said he had seen no evidence of interference in South Africa's courts.
Mr Zuma has said he will stand down as ANC leader only if he is found guilty of the charges - corruption, fraud, racketeering and money-laundering.
The controversial cartoon, which has dominated headlines in the South Africa this week, shows a woman, wearing a sash with the words “Justice System”, being pinned down by four figures.
They represent the ANC, the ANC Youth League, the South African Communist Party and the trade union organisation, Cosatu.
"Go for it, Boss!" they say to Mr Zuma, who was acquitted of rape charges in 2006, shown unbuckling his belt.
In a statement on Monday, the ANC, the SACP and the ANCYL called the cartoon "disgusting" and Cosatu has asked for an apology from the Sunday Times, which first published the cartoon.
But Mr Shapiro, known by his pen name "Zapiro", said he drew the "shocking image" of Mr Zuma as a "metaphor for what he's doing to the justice system".
He said that Mr Zuma had attempted to undermine the independence of the judiciary in order to get corruption charges "wiped off the slate".
"The aspect of Mr Zuma's own personal history of having been acquitted of a charge of rape is a secondary issue in this cartoon," he told the BBC's Network Africa programme.
"The primary issue is there's a figure about to rape the justice system with the help of his political allies," he said.
"It is derogatory, yes, it is demeaning, yes, but that's the intention."
Shadow of corruption
Mr Shapiro said he had been an ANC supporter all his life and South Africa had a "fantastic constitution".
"I feel that the ANC brought in the best things in this country and I think it is they that are threatening them," he said.
The shadow of corruption has been hanging over Mr Zuma for several years.
In 2005 he was sacked as South Africa's deputy president when his financial adviser, Schabir Shaik, was found guilty of soliciting a bribe on behalf of Mr Zuma and jailed for 15 years in connection with an arms deal.
Mr Zuma then went on trial, but the case collapsed in 2006 when the prosecution said it was not ready to proceed.
He was charged again last December shortly after winning a bitter campaign against President Thabo Mbeki to become ANC leader.
In February 2006, Mr Zuma was acquitted of rape in a separate case, though he was widely criticised for comments about sex and HIV/Aids.