Former president FW de Klerk has called Nelson Mandela a "brutal and unfair" opponent and said he was no "holy man", angering the ANC.
De Klerk, 76, who in 1993 won the Nobel Peace Prize jointly with Mandela, said that the anti-apartheid hero was a principled man of "stature and strength" but not faultless.
"I do not subscribe to the general hagiography surrounding Mandela," he said in a speech in Johannesburg on Monday, reflecting on their tense negotiations about democratic reforms.
"He was by no means the avuncular and saintlike figure so widely depicted today."
The ANC said De Klerk was poisoning South Africa with his remarks and could not acknowledge Mandela's bravery because the former president was black.
Keith Khoza, an ANC spokesman, said: "De Klerk should acknowledge Mandela and his achievements and understand that his time has passed as a president."
De Klerk spoke glowingly of former British prime minister Margaret Thatcher and her policies.
He praised her as a great leader who "took on the unions and won - and subsequently she took on the Argentinians and beat them as well".
"In all this she showed far greater determination and courage than any prime minister since Winston Churchill," De Klerk said.
De Klerk, who runs a charitable foundation, warned last month against ANC plans for a "second transition" amid concerns that South Africa's post-1994 constitution will be subverted.