JOHANNESBURG, SEPT. 22 -- President Frederik W. de Klerk should not be allowed to present the American people with an unqualified picture of himself as a man working for a peaceful settlement of South Africa's racial conflict, African National Congress leader Nelson Mandela said here on the eve of de Klerk's departure for Washington.

Speaking in an interview Friday, Mandela said Americans should be aware that the de Klerk government is playing a "double game," talking peace and negotiations with the ANC while conducting a war of destabilization against it.

Repeating accusations that security elements of the police and military form part of a "third force" fanning violence in black townships, Mandela said Americans should demand that the South African president explain why he is allowing this to happen.

Mandela said he did not object to de Klerk's trip to the United States and was not asking Americans to boycott him. "But the American people must know that apartheid is still firmly entrenched in this country and that our people are still being killed by the police," he said. "They must know that our people are still living under a very harsh political structure and that the police are still behaving as they did before the start of the negotiations."

There was a tinge of regret in Mandela's voice as he spoke of de Klerk, whom he said he had grown to respect and like during their frequent exchanges in the past nine months. In that time, they have moved from being deadly enemies to political partners in a quest to end the apartheid system of racial separation and lay the foundations of a new South Africa. Mandela said he had developed great admiration for the 54-year-old president but now feels that de Klerk is letting him down.

"I still regard him as a man of integrity, and I think he feels the same way about me," Mandela said. "We have developed enormous respect for each other, and we talk very frankly. I can call him at any time, I can get him out of bed or pull him out of cabinet meetings. We have that kind of personal relationship.

"I believe he and perhaps the majority of his cabinet are still as committed to the peace process as we are, but he has problems with elements inside his government, especially his security establishment which is riddled with right-wingers who are not with him at all, and he is not being frank with me about that."

He said the president is being "paralyzed" by a security establishment feeding him false information and so strong that de Klerk hesitates to challenge it directly.

"He is a very cautious chap and he is aware of his delicate position as far as whites are concerned," Mandela said.

Mandela said he regards as overwhelming and conclusive the evidence of security force involvement in the violence, both as part of a "third force" that is instigating trouble and by taking sides with members of Zulu Chief Mangosuthu Buthelezi's Inkatha Movement in the fighting. There are so many eyewitness accounts of this involvement that it is now the perception in the black community throughout the country, he said.

Mandela sounded aggrieved when he said de Klerk had not been more open with him in discussing why he could not act more decisively to stop this involvement.

"I have asked him very carefully, I have said, 'Tell me what your problems are, because if you do then I am in a position to help you, in the sense that I will report it to my executive that I think you have problems and that we must give you some space,' " Mandela said.

"But he keeps on telling me that his cabinet is united and that even his minister of defense {Gen. Magnus Malan} is behind him. I understand, of course, that being a politician he probably doesn't want me to know the differences inside his own organization," the ANC leader added.

Insisting repeatedly during the interview that he believes de Klerk is being misled by his advisers, especially in the security establishment, Mandela said he had several times seen the president look uncomfortable when confronted with information that contradicted what he had been given.

Citing one instance, when Mandela read him minutes from a meeting of the South African Communist Party, an ANC ally, that contradicted a report he had received from the security police, Mandela said de Klerk had been visibly agitated.

"Look, I know the man very well," he said. "He can be dignified. He is confident. He sits there very still and self-assured. But when I read him those extracts, he was fidgeting. You could see in his face that he had been given a totally different picture."

Despite his criticisms of the government and de Klerk, Mandela said he remains optimistic in the long term. He said he believes the negotiating process will survive its present crisis and ultimately succeed in laying the foundations of a non-racial South Africa.

"That is my personal view, because I am hammering de Klerk on this question. I am hammering him, and he is not indifferent," he said.