President Cyril Ramaphosa on Thursday told parliament that in his view it was treasonous to deny that apartheid was a crime against humanity, thereby publicly repudiating former president FW de Klerk.

Ramaphosa opened his reply to the debate on his State of the nation address by saying it was clear that apartheid was a crime against humanity even before the United Nations declared it such in 1973.

"They knew as they looked at this country that this was a country where a grave crime was being committed against the majority of the people against South Africa," the president said.

Ramaphosa added that there was not a single South African alive who was not touched by apartheid.

"I would go as far as to say to deny this is treasonous."

He was reacting to the bitter polemic that erupted after De Klerk compared apartheid to genocide and said he did not think it fell into the same category.

"Genocide is a crime," De Klerk said.

His apparent equivocation on the issue sparked a political storm, with the Economic Freedom Fighters disrupting the opening of parliament last week to demand that he be asked to leave the public gallery.

De Klerk on Monday issued an apology, and said he accepted that apartheid was indeed a crime against humanity.

He served as the country's last white president and then as deputy president in the democratic government formed by Nelson Mandela after South Africa's first all race elections in 1994.