Frederik van Zyl Slabbert, leader of South Africa's liberal opposition Progressive Federal Party, stunned the country's political leadership today by abruptly resigning to protest what he described as the government's insincere claims to be abandoning apartheid.

His unexpected announcement, which shocked the government as well as members of his own party, came moments after sharp splits developed in the ruling National Party when President Pieter W. Botha intervened in a parliamentary debate to rebuke his Foreign Minister Roelof F. (Pik) Botha for telling foreign correspondents that the government's plans theoretically could lead to a black presidency.

President Botha went on to endorse a conservative interpretation of the government's reform program given by the leader of the right-wing faction in his party, Education Minister Frederick W. de Klerk, which made it clear that the government was still committed to political participation on a racial basis.

Slabbert implied that this was the last straw in a process of disillusionment about the government's sincerity in wanting to end segregation and white-minority rule and about the role he could play in promoting negotiations between whites and blacks on the country's continuing crisis.

The dramatic gesture by Slabbert, coupled with President Botha's endorsement of the most negative interpretation of the reform program, are expected to negate much of the impact on foreign audiences that the government has hoped to make with Botha's "apartheid is outdated" speech of last Friday.

Slabbert's resignation is also a serious blow to the Progressive Federalists, the main white opposition party although they have only 27 parliamentary seats to the government's 125. As an Afrikaner liberal who has retained a close identification with the ruling white minority while building good relations with many blacks, including the exiled African National Congress, which he visited in Lusaka, Zambia, last year, Slabbert has acquired a personal status and influence far ahead of his party.

Botha's apparent backtracking today on what had been praised earlier as an encouraging declaration of intent led some local commentators to voice fears of another slide in the country's currency. The rand was beginning to recover after his hard-line speech of last August caused it to plunge to a record low as international bankers refused to roll over the country's $14 billion in short-term debts. A meeting with the creditor banks to seek an agreement on the repayment of these debts, which South Africa froze, is to be held in London on Feb. 20.

In his resignation speech, Slabbert implied that Friday's reformist statement by Botha, which was followed by a Madison Avenue-style advertising campaign, was intended primarily to impress the international bankers and local business leaders.

"It is not enough just trying to persuade the whites and foreign bankers that apartheid is being dismantled," Slabbert said. "What is much more important is to persuade the people who are the practical victims of apartheid -- and that you are not going to do with newspaper advertisements and double-speak statements."

Botha could "forget it" if he thought credible black leaders would be prepared to negotiate with him "on the basis of race-group politics," the opposition leader added.

Earlier in the day, the Cabinet held a crisis meeting to discuss different interpretations of Botha's Friday speech in which he said that apartheid was outdated and that the government was moving toward reform based on "the sharing of power between communities."

In the newspaper advertisements that began appearing at the weekend, a statement signed by Botha cited the formulation of "a single education policy" as a key point of this program.

On Tuesday de Klerk, who is also leader of the ruling National Party's biggest branch in Transvaal Province, explained to foreign correspondents that this policy was still one of racially segregated education.

At a similar briefing yesterday, Pik Botha, who is the leading figure in the government's verligte, or enlightened, wing, presented a much more liberal interpretation of the reform program, implying that the government was prepared to enter into open-ended talks with blacks.

Asked whether this might lead one day to a black presidency, Pik Botha replied, after a moment's hesitation, that this was "perhaps inevitable" provided a suitable way could be found to protect minority rights "without a racial sting."

The president made an unscheduled appearance in Parliament this afternoon to repudiate Pik Botha's statement and endorse de Klerk's interpretation.

"No minister has the right to compromise his party in this way," the president said of Pik Botha, revealing that the foreign minister had sent him a written apology, "which makes it possible for him to continue in his post."