Republican congressional leaders bluntly warned yesterday that legislation imposing economic sanctions on South Africa will be overwhelmingly approved, and that a threatened veto by President Reagan will probably be overridden unless he proposes strong alternatives.

The warning from Senate Majority Leader Robert J. Dole (R-Kan.) and House Minority Leader Robert H. Michel (R-Ill.) came as the president met with top foreign policy advisers at the White House to review events in South Africa and options for dealing with the sanctions legislation.

The Republican leaders privately relayed their concerns to the White House before they made public comments, officials said. Dole is to discuss it further today with White House chief of staff Donald T. Regan at a luncheon to review the fall legislative agenda, the sources said.

One informed official said Regan wants to avoid an embarrassing defeat for the president on South Africa because "he doesn't want to lose a big one" at the outset of the White House legislative drive this autumn.

Both Regan and national security affairs adviser Robert C. McFarlane are said by informed officials to be seeking alternatives to a politically damaging defeat.

These two senior officials have been told by White House legislative strategists that a veto override is certain unless the president attempts to "stake out some new ground," as one official put it.

This could include executive action to impose some limited measures, which the president finds acceptable, following a veto.

Officials said Reagan appears committed to vetoing the bill. He has said it would do more harm than good to South Africa's 21 million blacks. Supporters say the bill would push the white minority government in Pretoria toward dismantling apartheid, the system of rigid racial separation.

As Reagan began looking over options for dealing with the sanctions legislation yesterday, there were indications that a decision has not been made. A White House spokesman said there would be no comment on the meeting.

Dole, in an address scheduled for delivery last night to the Council on Foreign Relations in New York, said the Senate will "overwhelmingly" approve the sanctions legislation early next week. "If there was any question on that point it has been washed away by the flood of events in South Africa," Dole said.

"Though I share his reservations about sanctions as a way to implement foreign policy, the president must have some real administrative alternatives if a veto is to be sustained," Dole said.

"This is not a meat-cleaver sanctions bill," he added. "The sanctions it does contain are well focused to assure that the U.S. presence will not -- and equally important will not be seen to -- prop up the apartheid system."

Sen. Jesse Helms (R-N.C.), however, indicated yesterday that he will continue his filibuster of the bill, saying, "I think there will be some enlightenment going on."

Michel said the House would almost certainly override a Reagan veto. Noting that the bill passed by an overwhelming majority, Michel said, "I don't know how you turn yourself around on an issue like that" and uphold the veto after first voting for the bill.

On Wednesday, Chairman Richard G. Lugar (R-Ind.) of the Senate Foreign Relations Committee spoke to Secretary of State George P. Shultz by telephone in an effort to convince the administration that a veto would be unwise and unsuccessful, according to aides.

Lugar reportedly said he did not believe Reagan would pick up any votes by implementing some of the provisions of the bill while vetoing the bill as a whole. Lugar, who usually supports administration policy, is predicting about 80 Senate votes for the bill, far more than the 67 needed to override a veto.

The House already approved the sanctions legislation and a final Senate vote is expected next week.

Shultz faced persistent questioning about the administration's Africa policy yesterday morning in a 90-minute closed meeting with about 100 House Republicans.

"We asked him directly what the president thinks about conditions in South Africa and what he is going to do about the sanctions bill, but all we got was a history lesson about South Africa in response," one lawmaker who was present said.

When asked how many of the Republicans present voted against the House sanctions bill, opposed by the administration, only six lawmakers raised their hands.

At the State Department, spokesman Bernard Kalb said the administration view is that "sanctions will be counterproductive, hurting blacks in South Africa and stiffening white resistance to change." Kalb's remarks came several hours before Reagan's meeting with Shultz and other high officials about the administration's future course.