The South African government, defying warnings by the United States and other major Western nations, announced today that it is moving unilaterally to support the establishment of an internal administration for the disputed territory of Namibia, which it controls.

The new administration will be empowered to set up a council to draft a proposed independence constitution for the territory, President Pieter W. Botha told Parliament in Cape Town. South Africa is to maintain control over defense and foreign affairs under the new arrangement, according to Botha's announcement.

While Botha insisted the step would not affect the long-running negotiations over Namibia's future, western diplomats said tonight that today's action could not help but throw the delicate discussions into turmoil once again at a time when the United States has placed a new package of proposals on the table.

"It's going to cause a hell of a row all around," predicted one western diplomat, who foresaw adverse reactions from the so-called African "frontline" states that have played a key role in the negotiations, as well as from the United Nations and the Namibian independence movement, the South-West African People's Organization (SWAPO).

South Africa has ruled Namibia (South-West Africa), a former German colony, since World War I. Originally operating under a League of Nations mandate, South Africa has maintained control of the territory in defiance of the United Nations since 1966.

The United States and other Western powers, forewarned of today's move, have expressed fears that it threatens a deviation from South Africa's commitment to working with them for an internationally recognized independence settlement for Namibia. In making the announcement this afternoon, President Botha insisted that this was not the case.

Setting up an internal administration was compatible with the U.N.-approved settlement plan, Botha said, and South Africa would continue negotiating with the international community to try to achieve this settlement.

Any draft constitution produced by the new commission could be sumbitted to the constitutional commission envisioned in the U.N. plan if the negotiations still held any possibility of success, Botha said.

"The proposed arrangement . . . should be seen as an interim mechanism for the internal administration of the territory pending an agreement on an internationally acceptable independence," the president added.

Ambassadors of the U.S.-led western "contact group," which has been the mediator in the seven-year search for a negotiated settlement, lodged their governments' objections to the South African plan in a series of formal messages delivered to the director general of foreign affairs, Ray Killen, on Monday.

The messages, delivered by the ambassadors of the United States, Britain, West Germany and Canada, warned that these governments would regard any unilateral measures by South Africa to establish constitutional bodies in Namibia and transfer power to them as null and void.

The United States would regard any constitution worked out by the constitutional council as "without effect," according to the note presented by Ambassador Herman W. Nickel.

But South Africa rejected these objections in a response handed to the four ambassadors tonight.

"The South African government will, as in the past, consider any proposal of any political party which declares itself in favor of a peaceful solution to the problems of the territory," stated the South African response, which Foreign Minister Roelof F. (Pik) Botha released at a press conference after it was handed to the ambassadors.

Appearing with President Botha at the press conference, Pik Botha said he considered it "presumptuous" of the western governments to anticipate the president's announcement.

Both South African leaders argued at the press conference that the nature of the interim administration that South Africa set up in the territory was irrelevant to its commitment to continue negotiating for an international settlement.

"Surely South-West Africa must continue to be governed," news agencies reported President Botha as saying. "Either we govern it by means of the administrator general appointed by South Africa or we govern it by means of the leaders of the country."

Putting it more bluntly, with a reference to international claims that South Africa is illegally in control of the territory, Pik Botha said, "If the administrator general is illegally there, what does it matter if another body is also illegally there? It boils down to that."

Today's announcement came a day after South Africa completed its long-delayed troop withdrawal from Angola under an agreement with the Luanda government that U.S. mediators helped negotiate in February last year.

The troops had remained stalled 25 miles north of the border for nearly a year while South Africa tried to persuade Angola to convert a joint monitoring commission established under the agreement into a permanent border patrol to prevent SWAPO guerrillas from crossing into Namibia.

South Africa's sudden decision to drop this attempt and withdraw the troops, announced last Monday, has been interpreted in some quarters here as an effort to counter U.S. disapproval of the decision to set up an internal administration.

Why South Africa has chosen this moment to set up the internal administration remains a puzzle to most local observers.

Some think it may be an attempt to put new pressure on Angola to agree to the removal of Cuban troops from its territory, which is supposedly the last remaining obstacle in the way of agreement on an international settlement.

Others think it may be a deliberate attempt to provoke disapproving noises from Washington to offset criticism from President Botha's political opponents on the far right that he is acting under U.S. pressure.

Botha is known to have been irked by recent Reagan administration claims, made in response to congressional critics of its policy of "constructive engagement" with South Africa, that this policy has been responsible for cajoling Pretoria into introducing some reforms in the segregationist system called apartheid.

President Botha said in his announcement today that he was taking the step in response to requests from a group of Namibian parties known as the Multi-Party Conference because the situation in the territory could not be left to stagnate while the negotiations dragged on.

"The people of South-West Africa . . . cannot wait indefinitely for a breakthrough on the withdrawal of the Cubans from Angola," the president said.

In a veiled reference that seemed to carry a hint of a hidden agenda, President Botha added: "Should it eventually become evident, after all avenues have been thoroughly explored, that there is no realistic prospect of attaining this goal Cuban withdrawal , all the parties most intimately affected by the present negotiations will obviously have to reconsider how internationally acceptable independence may best be attained in the light of prevailing circumstances."

Botha said the members of the Multi-Party Conference had sought agreement on an independence formula from all Namibian parties, including SWAPO, but the nationalist movement recognized by the United Nations had failed to respond.

"The fact that SWAPO's views and the views of other South-West African parties are not included in the [Multi-Party Conference's] proposals is due solely to their own decision," he said.

South Africa caused similar consternation among the western mediators when it disrupted settlement negotiations by establishing a similar internal administration in December 1978. Botha did away with that administration in 1983 and placed Namibia under the direct rule of the administrator general.

South Africa's move comes just one month after the chief U.S. mediator, Assistant Secretary of State Chester A Crocker, announced a new package of proposals during a visit to South Africa.