South Africa, in an abrupt and dramatic change of tactics, marched into the U.N. General Assembly today for the first time in five years and asked to speak in the Assembly's debate on the territory of Namibia.

The credentials of the South African delegation had been rejected by the Assembly in 1974 on the ground that the apartheid system in that country make its government unrepresentative, and until today the South Africans had not challenged that action.

Yet when the Assembly reconvened its 33rd session this morning for a one-week debate on Namibia, South African representative J. Adriann Esksteen walked in and took the seat that had been empty for five years.

He was ostentatiously ignored as he listened to a series of African delegates call for economic sanctions against South Africa because of its refusal to accept a U.N. plan for the independence of Namibia.

Afterwards, Eksteen told reports that "we have a case to present to the Assembly, to refute these false accusations against South Africa." He said he had asked to address the Assembly tomorrow.

"If the Assembly is honest and fairminded, they will let us speak," he said. Esksteen admitted, however, that his right to speak may be challenged.

African diplomats indicated they would make such a challenge. "It is an anachronism to find South Africa in the Assembly hall," said Tanzanian Ambassador Salim A. Salim, "and the organization itself will be anxious to insure that anachronisms do not become a common practice."

The Africans are sure to have the votes to prevent Eksteen from speaking, whatever the legal niceties of the situation.

Both South Africa, in a letter to the United Nations from Foreign Minister Roelof F. (Pik) Botha, and the Southwest African People's Organization, the leading Namibian liberation movement, in a statement from its president, Sam Nujoma, maintained their willingness to enter into an agreemnt for Namibian independence drawn up by the United Nations. Each, however, blamed the other for stalematingthe negotiations through intransiegence and bad fiath.

Nujoma said that the recent installation of a legislative assembly in Namibia, without SWAPO participation, constituted a unilateral declaration of independence. This action, he argued, should stop the United States and its Western allies from arguing that negotiations on a U.N. supervised election are still viable.