JOHANNESBURG, JUNE 22 -- South African white extremists planned to assassinate African National Congress leader Nelson Mandela at the international airport here upon his return from the United States, according to evidence disclosed today by a man who says he was recruited to shoot him.

In a sworn affidavit published in the liberal Afrikaans-language weekly Vrye Weekbald, Jan Johannes Smith Jr. said he was recruited by the extreme right-wing Afrikaner Weerstandsbeweging (AWB), or Afrikaner Resistance Movement, to shoot Mandela with a special rifle from atop Jan Christiaan Smuts Airport. Smith's affidavit alleged that the group also planned to assassinate President Frederik W. de Klerk and other top officials.

Acting on evidence about the plot that Smith and the newspaper provided to authorities earlier this week, the police last night arrested 11 white, right-wing extremists, including the deputy mayor of nearby Boksburg. But by noon today, all had been released.

Late tonight, the head of police public relations, Maj. Gen. Herman Stadler, issued a statement saying the newspaper's "serious allegations are still being urgently investigated." He said it was still not clear whether groups or individuals were involved but that in his personal opinion "it seems to be emotional outbursts from individuals."

If there were an orchestrated plan to assassinate political figures, Stadler said, "I can assure you the police won't hesitiate to act against those who are guilty."

A police source, who asked not to be quoted by name, said the suspects had been released because "substantiation {of the allegations} was lacking" and "there was not sufficient proof to keep them in prison."

The AWB members were arrested under Section 50 of the Criminal Procedures Act, which allows police to hold suspects 48 hours for questioning, after which they must be charged or released. They were released within 12 hours.

By contrast, police have arrested and held incommunicado for months thousands of black anti-apartheid protesters for far lesser crimes under Section 29 of the Internal Security Act, which allows for virtually indefinite detention.

Smith and the newspaper provided the authorities with tapes and transcripts of his conversations with the assassination plotters.

In his affidavit, Smith said he had secretly made these recordings over a period of several months, and said he learned from the plotters that, in addition to Mandela, the AWB also planned to assassinate de Klerk, five cabinet ministers and a number of leftist white parliament members.

The group was plotting a right-wing takeover, starting with the seizure of parliament, the sabotage of power stations by throwing chains over high tension wires, setting fires in supermarkets to cause chaos, robbing banks to finance operations and smuggling arms into the country from Brazil, Smith's affidavit said.

The affidavit alleged that the deputy mayor of Boksberg, T.J. Ferreira, told Smith that he had discussed with the Conservative Party spokesman, Koos van der Merwe, the possibility of smuggling arms into parliament "bit by bit" so the AWB could "threaten parliament while it was in session using these weapons."

Van der Merwe was not available for comment tonight.

The AWB is not the only white extremist group alleged to be plotting against the government. The Boerestaat Party's deputy leader, Piet Rudolph, has declared "open war" on de Klerk's government and sworn to use "every means" to overthrow it.

Rudolph has gone underground after stealing arms from a South African Defense Force armory in Pretoria. The government has offered a $20,000 reward for information leading to his arrest.

Smith was a former part-time employee of the South African National Intelligence Service and the police security branch and currently works for the Boksburg City Council. But he had done investigative work for Vrye Weekblad in the past and had approached the newspaper after he was contacted by the AWB plotters, according to its editor, Max du Preez.

Du Preez and his newspaper have won considerable respect within the journalistic community for crusading articles against apartheid, the government's policy of racial separation, which denies blacks their political rights.

Smith is in hiding and under police protection, du Preez told a news conference today. Smith was scheduled to appear at the news conference, but the newspaper canceled his appearance. Du Preez said he fears for Smith's life.

Smith said in his affidavit that he was first contacted by Heinrich Beissner, a 76-year-old former World War II German pilot and regional AWB leader, on Feb. 2, the day de Klerk legalized all anti-apartheid groups, including the ANC.

After a series of meetings with Beisner and other AWB officials in the Boskburg area, Smith said, "they made a suggestion that I eliminate Nelson Mandela on behalf of the AWB. They explained to me that they did not want to use an AWB member for the elimination."

The AWB officials first "suggested that he be shot at a mass rally from a helicopter in the guise of a television crew covering the rally. I immediately rejected the plan as totally impractical," Smith stated.

Smith said that four days ago, Beissner contacted him to say he had the weapon "for the elimination of Mr. Nelson Mandela" and that it would be supplied to Smith by the AWB.

According to the affidavit, Smith was contacted Tuesday by a former South African Defense Force captain named Cornich, who told him about a new plan that called for Smith to assassinate Mandela with a .303 rifle, equipped with a telescopic sight and dumdum bullets, at the airport upon Mandela's return from the United States next month.

By this time, the newspaper had decided to inform the police about the plot and turned over its evidence, du Preez said. The assassination of Mandela, he said, was in an advanced stage of planning.

"They had planned it in detail. They had a very special rifle, with special bullets and a special telescope." he said. "They had worked out the distances."