JOHANNESBURG, JAN. 24 -- The Pan-Africanist Congress, South Africa's hard-line black nationalist group, today formally rejected a call by the African National Congress for a conference already accepted by the white government to discuss a nonracial constitution.

The decision seems certain to deepen political divisions among South Africa's 33 million blacks and possibly create difficulties for President Frederick W. de Klerk in an already complicated negotiating process.

A smaller black group, the Azanian People's Organization, earlier rejected the proposal by Nelson Mandela's ANC for an all-party conference. But Zulu Chief Mangosuthu Buthelezi and his Inkatha Freedom Party, key actors in current strife among blacks, have endorsed the idea.

While the Pan-Africanist Congress and Azanian group do not have as large an organized following as the ANC, their hard-line views on dealing with the de Klerk government and the constitutional process have won the sympathy of a significant segment of the black community, particularly among intellectuals and youth.

The group said it regarded the ANC's proposal as "no more than a reworking" of de Klerk's own attempt "to usurp the functions of an elected constituent assembly." It denounced the all-party concept as "a not very clever attempt" to substitute for a constituent assembly "an all-party congress cabal."

ANC leader Mandela proposed the conference on Jan. 8, calling it "the first step in the process leading to the adoption of a new constitution." The conference was to have defined broad principles, and Mandela hinted that it might be subsequently mandated to draw up the new constitution.

The ANC proposal was immediately accepted by the de Klerk government as a way of surmounting the issue of an elected constituent assembly. It opposes such a convention because the 5 million whites would be voted out of power at the start of the negotiating process.

The Pan-Africanist Congress also rejected "emphatically" the ANC call for power-sharing through an interim government, saying it was "not going to be party to legitimizing an illegitimate regime." It said the only negotiations it will participate in with the government were those necessary for setting up a constituent assembly based on one person, one vote.