The Southwest Africa People's Organization, Namibia's militant nationalist group, plans to reject the present Western peace plan for the territory and demands that a number of its key provisions be renegotiated.

A SWAPO brochure and policy statement, due to be published soon and devoted mainly to presenting the nationalist version of what happened during the South African raid in Kassinga in southern Angola May 4, states that "there can be no elections in which SWAPO can be expected to participate" under te existing proposal of the five Western powers.

In effect, the Namibian nationalists are telling the five Western capitals that if they cannot get South Africa, which has already formally accepted their plan, to accept major changes, they prefer to fight it out on the ground to the bitter end.

Whether South Africa will accept renegotiation of the plan remains to be seen, but observers here were skeptical that it would be willing to meet SWAPO's demands.

The SWAPO turndown and the continuing intransigence of both sides in the Rhodesian independence struggle are both setbacks to Western hopes of bringing about peaceful settlements in these two states bordering South Africa. An escalation of fighting would increase the likelihood of confrontation along East-West lines as in Zaire.

Specifically, SWAPO says the Western plan "cannot in its present form be regarded as final and definite. Some of its provisions must be renegotiated."

The thrust of the statement is that the five Western countries - the United States, Canada, West Germany, France and Britain - have failed in their effort to get South Africa to accept the basic provisions of U.N. Resolution 385 on Namibia (also known as Southwest Africa) and have come up with an election plan weighted heavily in favor of the South Africans and against SWAPO.

"It gives South Africa extensive powers to control and influence the outcome of the transitional process to Namibia's independence," the statement says.

The SWAPO brochure is entitled "Kassinga Massacre: Climax of Pretoria's All-out Campaign Against the Namibian Resistance." it is to be distributed throughout Europe and North America. It is being published in East Germany and contains pictures of Kassinga after the raid. A copy of the text was made available to jorunalists here, where SWAPO has its headquarters.

South Africa has administered Namibia, with a population of less than one million, under a League of Nations mandate since the end of World War I. It has agreed to grant the mineral-rich territory its indiependence by the end of this year. It has been involved with the five Western nations in negotiations for an international solution for the past 13 months.

South Africa surprisingly announced on April 25 during the U.N. debate on Namibia that it accepted the Western proposals for U.N.-South African interim rule of the territory and jointly supervised elections.

SWAPO appeared caught by surprise, refused to either accept or reject the proposals and then broke off talks with the five Western powers after the South African raid on Kassinga May 5.

Now, it is presenting its arguments for rejection, making it clear that the Kassinga raid and a South African crackdown on SWAPO members and their activities inside Namibia since late February were the decisive factors in it decision.

Furthermore, it blames the Western powers for failing to pressure South Africa sufficiently and for having "watered down" the U.N. resolution on Namibia that demanded, among other things, elections under U.N. supervision and control and South African respect for Nambia's territorial integrity.

"The fundamental flaw in these diplomatic efforts of the five has been the erroneous assumption that it was SWAPO that must be pressured, directly or through its friends, to accept to participate in elections which will merely be observed by the U.N. but which will in truth be controlled by South Africa," the brochure says.

The nationalists' specific objections to the Western plan include the following:

South Africa would be left with its "entrenched administration" over all of Namibia.

South Africa's paramilitary police force would remain intact and in charge of law and order during the transitional process.

South Africa would be allowed to have 1,500 combat troops stationed in north or central Namibia.

South Africa's claim to Walvis Bay, the main port, has been "tacitly conceded" by the Western powers and it would be allowed to have "more than a division" of its army at Rooikop in the bay area in addition to the 1,500 troops elsewhere.

"With all these essential elements of political and physical control thus firmly in the South African hands, there can be no elections in which SWAPO can be expected to participate," the brochure concludes.

The brochure describes the South African attack on Kassinga, 155 miles inside Angola, as "a cold blooded massacre of unarmed Namibia men, women, children, bed-ridden patients and aged people" at a refugee camp holding 4,098 people.

It said more than 300 of 568 primary schoolchildren in the camp were killed by bomb fragments when eights French-made Mirage jet fighters, attacked the refugees as they were gathering on the campus main square just after 7 a.m. The others, it said, took cover in trenches but were later hauled out and shot by the 780 South African paratroopers who attacked the camp.

Swapo's brochure quotes a foreign correspondent, identified only as Sara Rodriques, who visited the camp a day after the attack and reported seeing 122 bodies in one mass grave and 460 others in another.

Some reports have put the casualties at 1,000 dead and wounded.

The South African government said after the attack that SWAPO guerrillas had suffered "severe" casualties while five South African soldiers were killed.

The brochure says that the camp's school, medical clinic, kindergarten, garage and storage building were "reduced to burning rubble" and "nearly all" patients and medical personnel at the clinic were killed.

It also denies the South African claim that Kassinga was a SWAPO military headquarters. It says there were 300 soldiers stationed at Kassinga as a "camp defense unit" to gether with just two antiaircraft guns. The soldiers, it reports, put up a "heroically stiff resistance" killing 102 of the attackers and shooting down three South African planes.

Kassinga, the brochure argues, was not an isolated incident but "a climax of South Africa's recently stepped up waves of brute repression designed to liquidate or at least fatally weaken SWAPO both politically and militarily" so as to make way for a South African-imposed "puppet regime" in independent Namibia.