HARARE, ZIMBABWE, SEPT. 24 -- Oliver Tambo, the president of the African National Congress, today bluntly rejected as "propaganda" recent conciliatory signals from the South African government that it might be willing to open talks with ANC leaders on sharing power with blacks.

Tambo, in the group's strongest rebuff yet to peace feelers from South Africa, accused the government of "playing politics" with the issue of the release of imprisoned ANC leaders. He called on black South Africans to continue to struggle for the overthrow of the minority white government.

His remarks coincided with reports, confirmed by informed sources here, that on at least three occasions since July, emissaries from the South African government approached the ANC about the possibility of opening preliminary talks.

In an uncompromising speech here at an international antiapartheid conference, Tambo also rejected a South African proposal for the election of blacks to serve on a national council that would draft a new constitution giving the black majority a role in running the country.

"Of late, the Pretoria regime has been involved in a wide-ranging propaganda campaign whose aim is to give racial tyranny a new face and thus divert attention away from the ugly reality of the continuing system of apartheid," Tambo told delegates to the Conference on Children, Repression and the Law in Apartheid South Africa.

The conference was attended by 300 delegates, including nine ANC executive committee members in exile and 120 whites and blacks who flew here from South Africa. The ANC is outlawed in South Africa.

Besides Tambo, the senior ANC officials include Joe Slovo, a member of the national executive and head of the South African Communist Party, and Thabo Mbeki, the ANC's director of information.

"To hide this reality and to shift the focus of our offensive away from the objective of our advance toward people's power through struggle, the racist regime has been making all manner of noises about the issue of negotiations," Tambo declared.

He was referring to signals sent in recent weeks by Stoffel van der Merwe, South Africa's deputy minister for constitutional planning, that -- in the right political climate -- Pretoria might relax its position on refusing to talk with ANC officials.

In an interview last week, van der Merwe, a government leader in trying to negotiate power-sharing with blacks, said he would regard the ANC as a major participant in any talks. If the guerrilla group suspended violence, he said, the government might be able to start a process that could lead to its legalization.

Tambo, apparently trying to dispel expectations of early talks between Pretoria and the ANC, labeled as "complete falsehoods" the persistent reports originating from within his organization that attempts by Pretoria to establish indirect contact with the ANC already have been made.

"When and if the time comes that the apartheid regime feels compelled to talk to the ANC, it will have to come to us openly and not in secret," Tambo declared.

The South African government, which maintains that it is at war with the ANC, also has denied the reports. Informed conference sources said, however, that the ANC was approached three times by people claiming to bear messages from Pretoria.

The first, the sources said, was a member of a group of liberal white South Africans who met with ANC leaders in July in Dakar, Senegal. The sources said the delegate reported that before he left South Africa for Dakar, he was approached by an official of the National Intelligence Services who proposed a meeting between ANC leaders and a minister in {President Pieter W.} Botha's Cabinet.

The sources said the ANC sent a message back with the delegate asking what minister was being suggested and what he would be prepared to discuss. The ANC received no reply, the sources said.

The second attempted contact, according to the sources, occurred in Botswana when an ANC member in exile was approached by a businessman and friend from South Africa, who arranged a meeting with a South African security police official and a military officer.

The sources said the ANC member met the intermediaries, who conveyed an oral message that the government was proposing a meeting between an unidentified Cabinet minister and ANC leaders.

The third attempted contact took place in Lusaka, Zambia, when Kobus Jordaan, the government's constitutional development director, sent word through an intermediary that he would be willing to talk with ANC leaders if they were interested, the sources said. They said the proposal struck ANC leaders as "rather strange" and was turned down.

"There's no real willingness on the part of the Botha regime to sit and negotiate, so what's the point of talking?" a conference source said.

ANC leaders believe the signals are part of a strategy to persuade President Reagan and British Prime Minister Margaret Thatcher that, because of a new flexibility in South Africa's stance, economic sanctions by the United States and the Commonwealth of Nations are unjustified, the sources said.

If talks were held, the sources said, ANC leaders feel they would have to answer to their constituents in South Africa about why they were negotiating while blacks still were detained and the country was under a state of emergency.