South African air and ground forces launched a cross-border attack today against guerrillas fighting for independence in Namibia in a move that seems certain to derail next week's scheduled implementation of a United Nations peace plan.

Military authorities gave few details and did not even say where the attack occurred except to say it was across the northern border of Namibia, probably meaning Angola. Forces of the Southwest Africa People's Organization, which is carrying on guerrilla warfare in the South African controlled territory, are located in Angola and Zambia, both to the north.

The military would only say the attack was "limited." The only other time South Africa has publicly acknowledged a raid into Angola was last May, also at a crucial point in diplomatic negotiations. That raid, at Cassinga, was also termed "limited," but it was later established that more than 800 persons were killed, including women and children, according to Angolan authorities.

Any setback to the U.N. peace plan would be a blow to the Carter administration, which has held up the laboriously agreed upon plan for Namibia as a model for negotiated solutions to the racial conflict in southern Africa. The State Department recently said prospects were bright for completing a settlement this year.

Gen. Magnus Malan, chief of South Africa's defense force, said today's raid was necessary because SWAPO forces "had concentrated in the border area from where they launched an aggressive and defiant campaign of terrorism and violence directed mainly at the local population of South West Africa [Namibia]."

SWAPO activity has increased sharply since January, Malan said, in explaining why the raid was carried out.

The military attacks today coincided with the harshest verbal assault by South African Prime Minister Pieter W. Botha on the five Western powers who drew up the U.N. plan by brokering at long distance between SWAPO and South Africa.

SWAPO initiated its guerrilla war for independence 12 years ago against South Africa, which administers Namibia as a territory under an old League of Nations mandate no longer recognized by the international community.

Speaking in Parliament, Botha, who is also minister of defense, accused the United States, Britain, France, West Germany and Canada of duplicity and of breaking promises they made during their dealings with his government in negotiating the plan for a peaceful transition to independence in Namibia.

Botha said the five powers were guilty of "scheming behind the scenes" in giving their agreement to two clauses in the U.N. plan which South Africa objects to.

The South African leader said that a provision for SWAPO to set up guerrilla camps inside Namibia and the absence of any United Nations military monitoring of SWAPO bases in neighboring Angola and Zambia were unacceptable and would have to be modified for South Africa to cooperate further with the plan.

In a television interview tonight, when asked if it was "the end" if the two provisions were not modified, Botha replied, "this is so."

"This latest breach of an agreement was not the last straw. It was much more," the prime minister said. "If affected the basis of the most important aspect of the settlement plan namely that there should be peace and that peace should be visible."

"The worst shock was that the five Western powers supported these clear and deliberate departures and regarded them as a fair solution," Botha added.

Leaders of the opposition party, Colin Eglin, said the situation "calls for frank, man-to-man, top-level discussions of the Camp David style. I hope that somewhere in the Western world there will be a statesman big enough to take the initiative in this regard."

"Where do we go from here?" Botha asked in Parliament. "We stand by the settlement proposals which we accepted on April 25, 1978... Clearly stipulating that SWAPO personnel be restricted to their existing bases and that SWAPO's restriction to those bases be monitored."

The latest obstacles to an early implementation of the U.N. plan take place against a South African-imposed deadline for the U.N.-supervised independence elections. Botha today called attention to the fact that "it will not be possible" to delay the election beyond the end of September, 1979.

It was a widely held belief at the time of the Cassings raid that the aim of the South African government was political as well as military in that it hoped the attack would make SWAPO refuse to accept the U.N. peace proposals and thus bear the blame for the collapse of the Western initiative.

But, two months after the Cassinga attack SWAPO accepted the plan, largely it is believed, at the encour-agement of Angolan President Agostinho Neto who wants the U.N. settlement plan implemented in order to facilitate a reduction of the Cuban troops stationed in his country.

The Cuban forces along with East German advisors are training SWAPO guerrillas as well as helping the Angolan Army fight anti-Neto guerrilas who are supported by South Africa.