Assassination Plot Probed in S. Africa : Conspiracy: Informant alleges a right-wing scheme to kill Mandela, others in a nationwide reign of terror. The police detain, release 11 persons.

Police are investigating a plot by a right-wing white supremacist group to assassinate Nelson Mandela and several members of parliament and to undertake a nationwide program of sabotage and terrorism, officials said Friday.

Mandela was to be assassinated at the Johannesburg airport on July 18 when he returned from his international tour, according to an informant who said he had infiltrated the group of plotters after being approached to participate.

The informant was identified Friday as Jan Johannes Smith, a former police officer and agent of the National Intelligence Service, an arm of the government security apparatus.

Police said they had detained 11 persons Thursday night in connection with the plot, including the pro-apartheid deputy mayor of a conservative mining town on the outskirts of Johannesburg. All of the detained were released Friday.

The alleged plot included the assassinations of South African Communist Party leader Joe Slovo, an important supporter of the African National Congress, and two newscasters “as a symbolic obliteration of government policy.”

The plotters also considered trying to assassinate President Frederik W. de Klerk himself as the prelude to a coup, the informer said. Other assassination targets included Foreign Minister Roelof F. (Pic) Botha, Law and Order Minister Adriaan Vlok and Defense Minister Magnus Malan.

Security around De Klerk and his Cabinet was immediately strengthened.

“We are taking this seriously,” a spokesman for the Law and Order Ministry said.

When asked in New York about the police investigation, Mandela said he had not heard of the alleged plot but added: “I must inform you that I am so busy with positive work that I have little time for fears.”

He said that, as a member of the ANC, he is part of a team and that the group had carried on the struggle against apartheid during his 27 years in prison.

“If I am not there, the organization will continue . . . “ he said. “There are many highly respected leaders in my country.”

At the center of the plot, according to Smith, is the Afrikaner Resistance Movement, an increasingly vocal neo-fascist group opposed to De Klerk’s reform initiatives. Smith identified the ringleader as one-time Nazi SS Capt. Heinrich Beissner, 77, the leader of a branch of the movement east of Johannesburg with close ties to the movement’s leader, Eugene Terreblanche.

Since De Klerk began his campaign of political reforms in February, the movement has mounted a campaign of increasingly menacing rhetoric. At a rally last May, Terreblanche proclaimed: “The (Afrikaner Resistance Movement) and the entire right-wing spectrum in South Africa is arming itself and gearing up to use its weapons.”

The group has also been staging commando-style drills in public. But Smith’s allegation is the first time a plan of organized terrorism has been attributed to it.

Smith initially disclosed the alleged plot in an affidavit to the Vrye Weekblad, or Free Weekly, a liberal Afrikaans newspaper, which passed it on to police. The newspaper published the affidavit Friday.

A police spokesman, Capt. Piet Bothma, said the police would intently “follow up and investigate such stories. We can’t take chances with rumors of plot where people’s lives may be threatened.”

The allegations of an assassination plot emerged as South Africa was absorbed in another right-wing drama--this one involving a fugitive former Pretoria city councilman who has been in hiding since April 13, when he claimed to have masterminded the theft of weapons from an air force arsenal.

The fugitive, radical right-winger Piet Rudolph, surfaced this week in a videotape sent to the Afrikaans daily newspaper Beeld, in which he proclaimed in Afrikaans: “We will use every means at our disposal to fight the De Klerk government, the overthrow of which is of the highest priority to us. It is now open war.”