President Reagan intends to take executive action Monday imposing limited measures against South Africa in an effort to prevent an embarrassing defeat in Congress over legislation imposing tougher economic sanctions on Pretoria, administration officials said yesterday.

The Senate is expected to approve sanctions legislation next week, and Reagan will probably veto it, but officials said the White House is planning a major effort to sustain the veto. They said the early announcement next week, before the final Senate vote, is a tactical move to try to forestall a congressional override of Reagan's veto.

Reagan is expected to argue that he has taken sufficiently strong action, and that the United States must continue with its policy of "constructive engagement," seeking to influence the government of President Pieter W. Botha through dialogue and not coercion.

"I think it's the only thing that's shown any sign of improvement in that whole situation as yet," Reagan said yesterday of the policy toward South Africa, which has come under intense criticism in Congress and elsewhere.

Also yesterday, Reagan told reporters that he had "carelessly" said in a radio interview two weeks ago that segregation in public places had been "eliminated" in white-ruled South Africa, where the system of rigid racial segregation known as apartheid has been at the center of a deepening cycle of violence the last year.

On the sanctions legislation, presidential spokesman Larry Speakes said Reagan is seeking more information and consulting with allies and Congress before making a final decision. But officials have said he is choosing for the Monday announcement provisions out of the pending legislation, which the House has approved.

Previously, U.S. officials have said two such provisions would be forbidding sales of computers to South African government agencies that administer apartheid and prohibiting U.S. government loans to businesses there that do not follow equal employment guidelines.

Senate Majority Leader Robert J. Dole (R-Kan.) said yesterday that "we might sustain a veto" if Reagan implements some of the proposals in the sanctions legislation. "I suggested they take a look at what's in the bill and implement some of its provisions," he said.

But Dole added that he told White House chief of staff Donald T. Regan at a luncheon meeting that the Senate will approve the sanctions legislation next week "with a large vote." In a brief question-and-answer session with reporters at the White House yesterday, Reagan was asked about his remark to WSB Radio in Atlanta Aug. 24 in which he said of South Africa, "They have eliminated the segregation that we once had in our own country -- the type of thing where hotels and restaurants and places of entertainment, and so forth were segregated -- that has all been eliminated."

Reagan said yesterday he did not believe that and "I didn't intend to say that."

When he made the remark, "I did know that all the people that have been coming back here and have been reporting to me on how widespread this was, and I'm sorry that I carelessly gave the impression that I believed it had been totally eliminated. There are some areas where it hasn't," he said.

Asked why he was "so misinformed about the state of the situation" in South Africa, Reagan responded, with a tone of indignation:

"I was not nearly as ill-informed as many of you made it out that I was. I may have been careless in my language in that one thing, but I was talking about improvements that actually do exist there and have been made. But as I say, I know that segregation has not been eliminated totally and in some areas there's been no improvement. But there has been a great deal of improvement over what has ever existed before."

When a reporter pointed out that the nation's 21 million blacks are still denied a vote, Reagan said, "No, no, no. I was talking about the specific things of segregation, of labor and the new things that have taken place with regard to labor and things of that kind."

Reagan has often threatened to veto the sanctions legislation on grounds that it would hurt South Africa's blacks. But officials said the president is prepared to impose some limited measures now, while still pursuing his "constructive engagement" policy.

"This is not the adoption of a punitive approach," a White House official said of the measures Reagan is expected to announce next week. The official said Reagan's move is "not the beginning of a departure" from the policy of trying to end the violence in South Africa and bring about a dialogue between all groups on the country's future.

One of the questions facing the administration is when and under what circumstances to send U.S. Ambassador Herman Nickel back to Pretoria after an absence of nearly three months. Nickel was brought back for "consultations" as a sign of displeasure with South African policy, especially the South African guerrilla raid on Gulf Oil Co.'s installation in Angola