HARARE, ZIMBABWE, SEPT. 25 -- African National Congress President Oliver Tambo today challenged the South African government to follow up recent conciliatory signals with concrete steps that could lead to formal negotiations with the ANC.

Tambo also challenged South African President Pieter W. Botha to explain to the white electorate the need to negotiate with the ANC, which is outlawed in South Africa and which Pretoria until now has portrayed as a communist-inspired terrorist organization.

Confirming publicly for the first time that emissaries from the South African government have approached the ANC about the possibility of preliminary talks, Tambo said he found it difficult to reconcile Pretoria's "vicious attacks" on the organization with its recent conciliatory overtures.

But in remarks to reporters at an international conference on apartheid, the ANC leader said, "We need to watch it and see where the maneuvers are leading."

During the conference, which has brought together the largest gathering yet of top ANC officials and white and black antiapartheid activists from South Africa, Tambo is known to have revealed an intense interest in recent conciliatory statements by some South African officials.

Tambo is known to be particularly curious about suggestions by Botha's point man on negotiations with blacks, Deputy Minister for Constitutional Planning Stoffel van der Merwe, that the ANC's strategy of violence originally may have been justified and that a suspension of violence now could start a process leading to legalization of the organization.

Tambo expressed some wariness of the indirect approaches by South African emissaries that began in July, saying in response to a question at his news conference, "It is not for us to follow up. It is for the people who made the approaches."

According to informed conference sources, three approaches have been made to the ANC -- in Dakar, Senegal, during a meeting of liberal South African whites and ANC officials; in Botswana; and in Lusaka, Zambia, where the ANC has its headquarters.

In the contacts, persons claiming to be intermediaries for the South African government asked whether the ANC would be interested in meeting with a member of Botha's Cabinet. Because the minister was not identified and the subject of the proposed talks was not specified, the ANC rejected the overtures, the sources said.

In two of the approaches, they said, there appeared to be involvement by the South African National Intelligence Services, which Botha closely monitors.

Tambo made it clear today that any contacts -- if they were to take place -- would have to be in the open and that the agenda for talks would have to be made public.

Tambo said his constituents are sensitive about secret talks so long as South Africa remains under white domination and blacks are still imprisoned.

"We are very particular about things we hear from the other side. We make sure that nothing will go wrong with the people's confidence in the ANC," he said.

But in his first public reaction to what appear to be emerging as hesitant, carefully guarded signals by both sides, Tambo offered Pretoria a potential formula for getting talks started.

Tambo recalled that the Commonwealth's Eminent Persons Group, which visited South Africa in 1986, recommended "proposals which might be done to clear the way for possible negotiations."

The proposals that he cited were dismantling apartheid; releasing imprisoned ANC leader Nelson Mandela and other political prisoners; lifting the state of emergency; and legalizing the ANC and other black nationalist groups.

"None of these has happened," Tambo said, in a clear challenge to Pretoria to take the initiative on starting talks. When asked to speculate where the South African signals are leading, the ANC president replied, "I wouldn't know. It wouldn't help {to speculate}."

Tambo complained that Botha's ruling National Party, in an effort to rally white support in parliamentary elections in May, sought to destroy the ANC's international image by portraying its leaders as "gruesome murderers."

"The white electorate in South Africa still retains some hair-raising notions about the ANC. If {government officials'} attitudes have changed, Pretoria should go back to the electorate and tell them, 'The ANC is not really the monster we told you it was,' " Tambo said.