The President's office has taken time to slap down a constitutional law professor who called President Jacob Zuma a gangster.

Pierre de Vos, from the University of Cape Town, was quoted as saying about the appointment of Menzi Simelane as director of public prosecutions that it shows an utter disregard for the Constitution and the law.

"It is nothing more than the actions of a gangster hell bent on protecting himself and his cronies. I feel ashamed that I have given our President the benefit of the doubt for all these months," De Vos said and repeated the remarks on his blog.

The Presidency said on Tuesday that it finds the statement 'shocking and despicable'. Such utterances "clearly go beyond the standards of decency and intelligent debate."

The presidency, breaking out the capital letters, insists that "reference to the Head of State of the Republic of South Africa as a 'gangster' does not show respect for the incumbent as an individual and also insults the Office that he holds".

"The Presidency welcomes issue-based debate and President Zuma encourages debate and the views of all, even those who disagree with him vehemently," the statement issued by the president's office said.

"It is our opinion that it is possible for constitutional law experts to differ with government and disagree without being disagreeable, and without making inciting statements or insulting utterances.

"President Zuma is known for encouraging mutual respect for political opponents and people who differ with him and for always observing diplomacy and decorum. It would not be asking too much to encourage Professor de Vos to embrace maturity and decency in expressing his anguish whenever the need arises."