Zephania Mothopeng, 77, who broke away from the African National Congress to help form and lead the militant Pan Africanist Congress, died yesterday in South Africa. He had been suffering from cancer and pneumonia. Mr. Mothopeng lived his last years a few blocks from fellow black nationalists Nelson Mandela and Walter Sisulu, but his militant stance kept him far from the more moderate ANC leaders. His release from Johannesburg's Diepkloof Prison in November 1988 preceded reforms that included freedom for the ANC leaders and government promises to include blacks in a new constitution. Mr. Mothopeng backed armed struggle and opposed talks with the government intended to pave the way for a new constitution. Power should be returned to the indigenous Africans, Mr. Mothopeng said. "The question is one of the repossession of our land from foreign colonial oppressors," he said in an interview this year.