JOHANNESBURG, JUNE 1 -- South Africa's Inkatha Movement, a predominantly Zulu organization led by Chief Mangosuthu Buthelezi that has long been viewed as a rival to the African National Congress, was dealt a major blow today when its secretary general, Oscar Dhlomo, resigned.

Dhlomo, 47, considered Buthelezi's right-hand man and likely successor, gave no reason for his resignation beyond saying he wanted to devote more time to his family and other interests. But he is known to have been at odds with Buthelezi recently as a result of the Inkatha leader's bitter clashes with the ANC, South Africa's most prominent black nationalist group, and with ANC allies.

Dhlomo said he would leave Inkatha and also would give up his government positions in the segregated black homeland of Kwazulu on June 30. He is minister of education and culture in Kwazulu and a member of its legislative assembly.

Political analysts said tonight that Dhlomo's resignation would be a severe blow to Inkatha at a time when the organization is losing support within its main power base, the southeastern province of Natal, to the ANC and its allies, the United Democratic Front and the Congress of South African Trade Unions.

Black opponents of the Inkatha Movement have accused it of being soft in its opposition to South Africa's apartheid policy of racial separation and have criticized Buthelezi as authoritarian. Some white South Africans view Inkatha as a relatively moderate organization because of Buthelezi's rejection of socialism and opposition to the ANC's armed struggle against the government. In a meeting with President Bush in February, Buthelezi urged the United States to lift sanctions against South Africa.

A former university lecturer with a doctorate in education, Dhlomo was seen as an intellectual in Inkatha, a largely rural-based, tribal organization that traditionally has dominated South Africa's 7 million Zulus. He played a key role in Inkatha's dealings with white businessmen, foreign visitors and other political organizations.

Buthelezi issued a brief statement tonight expressing regret at Dhlomo's resignation and giving no hint of tension between them.

He paid tribute to Dhlomo's work in Inkatha and as head of the Natal-Kwazulu Indaba, a conference of representatives of the Kwazulu homeland and the white provincial council of Natal that developed a plan for joint administration of the province. Despite wide support, the plan was rejected by the Pretoria government.

Dhlomo has been a member of a joint peace committee seeking to end bloody fighting in Natal between Inkatha members and Zulu supporters of the ANC and its allies. The fighting has claimed more than 3,500 lives in the last three years.

Committee members representing the United Democratic Front, a coalition of anti-apartheid groups, have said they developed a good relationship with Dhlomo and regarded Buthelezi as the chief stumbling block in the peace efforts.

There was no immediate indication whether Dhlomo would consider joining the ANC or an allied organization, although UDF leader Popo Molefe said tonight he did not believe a man of Dhlomo's abilities could remain inactive during this crucial period in South Africa.

"A man of his experience and standing will not be able to have a long rest before he is approached to take up an influential position in politics," said Peter Gastrow, a member of the white liberal Democratic Party from Natal Province.