Once again, a U.S.-supported United Nations plant to prevent a minor African were here form becoming a major one, has run into a snag that could cause its collapse.

South Africa appears ready to reject the plan because it would allow the guerrillas who have been fighting here for 12 years to establish a formal military presence in Namibia (Southwest Africa).

This reportedly was the position communicated by South Africa's Prime Minister Pieter W. Botha and his foreign minister, South Africa's Pik Botha, when they met here today with the local governing body that is dominated by their political allies.

Both South Africa and the Sovietarmed Namibian nationalist movement, the Southwest Africa People's Organization (SWAPO), have been rquested by U.N. Secretary General Kurt Waldheim to convey by Monday their acceptance of the plan -- which calls for a cease-fire on March 15 and stationing at that time of peacekeeping forces in Namibia.

Both SWAPO and South Africa have been reluctant participants in the negotiations that led to the U.N. plan and both are skeptical that their best interests are served by agreeing to the U.S.-supervised cease-fire and elections called for inthe plan. South Africa appears to fear that SWAPO, which it considers communist, will win the elections. SWAPO appears to fear that it will fail to sweep the polls and will be forced to form a coalition government with a party backed by South Africa. Both parties, however, would like the other to reject the plan first.

If past performances inthe U.S.-led negotiating process -- begun in 1977 -- is an indication, South Africa is most likely to say it accepts the plan only on the condition that the clause permitting SWAPO guerrillas to be based in the territory is altered or deleted. This could then amount to a rejection of the plan if Waldheim does not accede to the South Africans' wishes.

Such a rejection would present the Carter administration with a dismal filure in its efforts to negotiate peaceful solutions to conflicts in southern Africa -- efforts which have gottennowhere in the Rhodesian war.

Assistant Secretary of State Richard Moose declared in late December that the Namibia negotiations were "the most successful U.S. undertaking in Africa" last year.

For the first time, South Africa is joined in its objections to the plan by two centrist political groups within Namibia that inthe past have criticized the South African government's positions in the negotiations.

"We are against any political party with armed men insider the country, even if they are confined," sai Andreas Shipanga, leader of SWAPO-Democrats, a moderate dissident splinter group from SWAPO. An alliance of moderate political parties, the Namibian National Front, also objected, saying the provision to permit the SWAPO-military presence "would be entirely contrary to the spirit of the Western proposals."

The National Front has proposed that guerrillas who step forward at the time of the cease-fire be given the choice of returning to SWAPO's established bases in neighboring Angola or giving up their arms to the U.N. peacekeeping force and returning to civilian life.

Despite obejctions, the political parties inside Namibia that have opposed South Frican rule in the past do not reject the U.N. plan out of hand.

"I agree with their objections to a SWAPO military presence, said one black politician, "but I don't agree with using it as an excuse to get out of the U.N. plan. All blacks in this territory want U.N.-supervised elections."