South Africa installed a new internal administration in Namibia today amid a combination of pageantry and violence, staging an elaborate inauguration ceremony on one side of this picturesque capital while tear-gassing dissenters in a black area on the opposite side.

The inauguration featured the kind of pageantry normally reserved for declarations of independence, with a mechanized military procession, displays of stunt flying, parachuting, torch-bearing runners, fireworks, and a 21-gun salute for the new Cabinet.

While this was taking place, police, headed by a tough counterinsurgency brigade called Koevoet, maintained a massive presence in Windhoek's main black township of Katutura, where a day-long rally of dissenters was held. As the dissenters left a soccer field toward evening, the police dispersed groups of them with batons and tear gas. Six reportedly were hospitalized with baton injuries.

South African President Pieter W. Botha, who performed the inauguration ceremony, repeated earlier assurances that South Africa intends the new Namibian administration purely as an "interim mechanism for the internal administration of the territory." He said South Africa remains committed to seeking an internationally acceptable independence settlement for the territory through U.S.-led negotiations.

Despite these assurances, the United States and other western intermediaries have condemned the move, which informed sources here see as part of a broad South African strategy to bypass a United Nations program for the territory's independence.

U.N. Secretary General Javier Perez de Cuellar declared the new administration "null and void" today and urged all states to deny it recognition, United Press International reported.

South Africa has controlled Namibia's affairs for 67 years. In 1966, the United Nations passed Resolution 435, calling for an end to South African control, and later formed a U.N. special council to administer the territory until independence. South Africa has been formally committed to the U.N. measure for six years, but has largely disregarded it because of the role it gives the United Nations as supervisor of the transition to independence and because South Africa says it favors the main Namibian nationalist movement, the South-West African People's Organization (SWAPO).

The new South Africa-installed administration does not include representatives of SWAPO, although it does include some with African nationalist backgrounds, notably Moses Katjiuonga, 43, leader of the South- West African National Union (SWANU). Another of the new administration's leaders is Andreas Shipanga, one of the founders of SWAPO who later fell out with its leader, Sam Nujoma, and formed his own breakaway party, the SWAPO-Democrats.

South Africa is handing over powers of self-government to the new administration, except in foreign affairs, defense, and internal security.

Western critics of South Africa's latest move accuse the country of trying to impose a "regional settlement" of its own.

Botha said today that South Africa would do nothing irreconcilable with the U.N. plan as long as there was a realistic prospect of success in the present negotiations to bring about the withdrawal of the estimated 25,000 Cuban troops in Angola, which, according to South Africa, is the only remaining obstacle to the U.N. settlement.

But, Botha added, the people of Namibia could not wait indefinitely for a breakthrough on this so-called "linkage" issue, jointly introduced by the United States and South Africa four years ago.

"Should it eventually become evident, after all avenues have been thoroughly explored, that there is no realistic prospect of attaining the goal, then all 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.

Observers see this as the opening to introduce South Africa's internal strategy, bypassing the U.N. plan. South Africa, these observers say, wants a plan that would reduce the U.N. role to a nominal presence while giving the main supervisory functions to a regional force made up of itself and the so-called "front-line states" of southern Africa.

The plan also would contain mechanisms that, while not preventing a SWAPO takeover, which most observers think is inevitable, would dilute SWAPO's radicalism by forcing it into an alliance with other parties and imposing a more restrictive constitution.

Botha listed certain "ground rules" that he said were gaining wider acceptance in southern Africa, including one that "these problems should be solved on a regional basis by the leaders of the region themselves."

Other South African officials speak with the same ambiguity, insisting both that the government remains committed to the U.N. plan and that the new internal administration is necessary because Namibian politicians are becoming frustrated by the lack of progress in the negotiations. These officials suggest that the new Namibian administration might open the way to a "new initiative."

As one U.S. diplomat put it, "At the minimum, this new administration is occupational therapy for unemployed politicians. At the maximum, it is something intended to force SWAPO to accept some kind of political deal with the internal parties."

Most observers here in the territorial capital describe it as the latter, and that South Africa's plans for forcing SWAPO's exiled leadership to accept the alternative deal include stepping up its aid to Jonas Savimbi's rebel movement in Angola, in the hope that his position may be so strengthened that he can negotiate a place in the Angolan government.

Some believe the recent attempt by a South African commando unit to sabotage a major oil installation in Angola's northernmost province of Cabinda was an attempt to help Savimbi.

If Angola's leftist government can be forced to accommodate Savimbi, so the thinking goes, Savimbi would increase Angola's pressure on SWAPO to abandon its increasingly unpromising attempt to win independence for Namibia through guerrilla warfare, and switch to a political campaign through the new Namibian self-government structure.

If SWAPO were to do that, it is thought that South Africa would drop its insistence on Cuban troops leaving Angola as a precondition for Namibian independence. In effect, that would finesse the U.N. plan.