JOHANNESBURG, AUG. 7 -- Moderates throughout South Africa hailed the cease-fire announcement by the African National Congress early this morning as a breakthrough toward a negotiated settlement of this country's racial conflict, while extremists on both the white right and black left condemned the accord as worthless and a sellout.

Andries Treurnicht, leader of the far-right Conservative Party, described the agreement -- reached in a 15-hour bargaining session between the ANC and the government of President Frederik W. de Klerk -- as "untenable and illegal."

"The ANC is not an alternative or sovereign power that can make agreements with the legitimate power in the country," he told reporters in Pretoria today.

Treurnicht said the ANC's announcement that it was suspending its 29-year armed struggle against apartheid was meaningless, because the black nationalist movement had not renounced the use of violence and would revert to it if the negotiations broke down.

"The ANC is only interested in the surrender of power and not in its sharing," Treurnicht said. "It's a case of talk or fight. If the talks don't bring about the required result, which is the surrender of power to it, then the fighting continues."

A prominent black militant, Zeph Motopeng, leader of the Pan-Africanist Congress, accused the ANC of betraying the black cause. He said Nelson Mandela, deputy president of the ANC, had won no meaningful concessions from the government in return for his suspension of the armed struggle. All de Klerk had done was make vague promises to review repressive legislation some time in the future, Motopeng said.

Motopeng said at a press conference in Johannesburg that his organization rejected the agreement and would not be bound by it. The Pan-Africanist Congress (PAC) "will continue the struggle in all forms, including armed struggle," Motopeng said.

The Conservative Party and the PAC are both minority parties in their respective racial constituencies but are believed to be growing rapidly as racial extremists turn away from de Klerk's long-dominant National Party and Mandela's ANC, which is the oldest and most broadly based black nationalist movement in South Africa.

Results in recent by-elections suggest that if a general election were held now under the present constitution, in which white votes are decisive, the Conservatives could oust the de Klerk government. The strength of the PAC is more difficult to gauge, but many political analysts agree that the growth of the two extremist wings poses the most serious threat to the negotiating process initiated by de Klerk last February.

To counter it, these analysts say, a strong political center would have to emerge as a base for a post-apartheid South Africa. They say the ANC's reluctance to abandon its armed struggle aroused strong feelings in the white community, which has prevented a political center from developing around the National Party and the ANC.

Today, a wide cross-section of organizations and individuals of all races expressed approval of the ANC's decision to suspend its use of violence.

Welcoming statements came from businessmen, lawyers' organizations, church bodies and political groups. A prominent Afrikaner business organization, the Handelsinstituut, called the ANC's decision "especially encouraging" and said it indicated "a commitment to a process of peaceful negotiation that can only lead to business confidence in the future of South Africa."

The South African Council of Churches, which represents most of the country's Christian denominations, said the accord would "open the doors to all South Africans to participate in the process of negotiating a new constitution."

The next stage in the complex negotiating process, according to Thabo Mbeki, a member of the ANC's negotiating team, will be for the ANC and the government to agree on who should be consulted about the format for negotiations on the shape of a new constitution for South Africa.

The ANC wants one-person-one-vote elections to an assembly that would draft the new constitution, the process used in neighboring Namibia, which this year gained independence from South Africa. The government opposes this because the country's five-to-one black majority would relegate it to a minority position at the negotiating table.

The government wants all recognized parties to be represented, but the ANC objects that many of these do not have authentic support. It says only an election can establish their true strengths. Before a compromise can be sought, there must be agreement on who should do that negotiating.