South Africa was bracing itself for the worst today as both the United Nations and the Carter administration began separate deliberations over what measures to take in response to last week's massive crackdown on black and white dissent in this racially unsettled nation.

There is a feeling here of an imminent confrontation between the United States and South Africa over the government's latest efforts to crush the black power movement, a confrontation that is likely to have serious repercussions for the policies of both countries throughout Africa.

In fact, the Carter administration appears to be facing a major crisis of credibility in its carefully nurtured African policy. If it fails to take any meaningful action against South African, black African nations as weel as the black population here are certain to question the substance and direction of American diplomacy toward this bastion of white rule.

On the other hand, if the U.S. government does back economic sanctions or other measures unacceptable to South Africa, it may well lose South Africa's crucial support for U.S. initiatives to find a peaceful solution to the Namibia and Rhodesia disputes.

The net results of this could be to push all of southern Africa toward the racial conflagration that the United States and other Western powers are so anxious to avert.

Prime Minister John Vorster and his top ministers have been preparing the 4.2 million whites psychologically for the possible imposition of diplomatic and economic sanctions on South Africa for months now and particularly for the prospect of strong pressure from the United States.

In fact, South African Foreign Minister R.F. "Pik" Botha told this jittery nation 10 days ago that the United States now constituted a bigger threat than the Soviet Union and said , "The Carter Administration has decided that the white people of South Africa have to be ploughed under."

The government's strong anti-American propaganda, which began months ago in reaction to various pro-black statements by top U.S. officials, has now dovetailed into the ruling National Party's campaign to whip up support before national elections Nov. 30.

The government apparently hopes that by promoting the image of besieged nation it can draw all whites into the National Party's Laager, the enclosure of ox-drawn wagons used by the Afrikaner pioneers to ward off African attacks.

Hardly a day goes by without one minister or another making a vitriolic attack on the Carter administration and affirming the National Party's readiness to "go it alone" if necessary in the struggle to preserve white rule here. The slogan of one man, one vote voiced by Vice President Mondale at his meeting with Vorster in Vienna last spring is constantly cited by Nationalists a proof that the United States is out to do the whites in.

This thesis has now been twisted to blame the United States for the present crisis in black-white relations here that has made the prospects for peaceful change so problematica.

Replying to President Carter's statement that Last week's crackdown on dissent here was a "retrogressive step," Foreign Minister Botha said that such statements made change "infinitely more difficult."

Reports in the local newspaper of mounting U.S. pressure on South African he told the Rand Daily Mail, "have the effect of encouraging radical militants in southern Africa with the danger that what is seen to be American policy could ... cost the lives of many thousands of non-Americans, both black and white alike."

He said that South Africa would not be held hostage by the threat of sanctions, adding "the more we are threatened, the more we will be prepared to resist."

This theme of defiance was repeated today over the state-controlled Radio South Africa. It said the government no longer cared whether the United States used its veto in the United Nations to prevent sanctions being imposed on South Africa.

The Western use of the veto, it asserted was not being done out of friendship for South Africa anyway in order to protect Western trade and other vital interests here.

Some local observers have become convinced, however, that the Vorster government does not really believe the United States is ready to apply all that much diplomatic or economic pressure. This is because the Western search for a peaceful solution to the Namibia and Rhodesia disputes, in which the Carter administration has taken a leading role, has reached a delicate juncture where it could easily fail without South African support.

These observers believe Vorster and his minister took this very much into their calculations in deciding last week to go ahead and ban 18 black and white oppositions groups and detain scores of black leaders.

Interestingly enough, a number of moderate black leaders here also seem to doubt that the United States is prepared to sacrifice its economic and political interests in South Africa for the sake of the black cause.

Should the Carter Administration take only "cosmetic measures" now it is likely to strengthen black opinion that the United States is still not serious about forcing the South African government to abandon its apartheid policy.News agencies reported these other developments involving South Africa:

West Germany has recalled its ambassador to South Africa "for constultations" on Pretoria's banning last week of two black newspaper and 18 antiapartheid organizations, the Foreign Ministry announced in Bonn.

In London, British Foreign Secretary David Owen summoned Britian's ambassador to South Africa. Sir David Scott, to his office for consultations on recent events in South Africa. Scott was already in Britain on leave when the crackdown on dissent took place in South Africa last week.

Both the U.S. and Dutch ambassadors to South Africa were recalled by their governments last week.