Senate Republican leaders, dismissing President Reagan's pleas, said yesterday they will defy the White House and press for enactment of tougher economic sanctions against the white minority regime in Pretoria.

Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) endorsed a series of "very targeted, specific" sanctions, including suspension of landing rights to end air traffic between the United States and South Africa, a halt to U.S. consular services there and new restrictions on South African accounts in U.S. banks.

"I think we should take action, and for Congress that means legislation," Lugar said.

The comments of Lugar and others suggested that a coming congressional clash on South Africa policy will be over how tough sanctions should be. It is likely to pit moderate Republicans, who now favor at least limited sanctions, against liberal Democrats, who are calling for a complete withdrawal of U.S. commercial interests. Despite Reagan's call to "resist the emotional clamor for punitive sanctions," support for the administration's policy appears to have dwindled.

Sen. Nancy Landon Kassebaum (R-Kan.), chairman of the African affairs subcommittee, said she was "deeply disappointed that the president chose not to announce a new direction for U.S. policy in South Africa."

"I agree with the president's strong opposition to apartheid and with his equally strong opposition to total U.S. withdrawal from South Africa," Kassebaum added. "However, I do not believe the United States can passively accept the status quo. It is time to make clear to the government of South Africa that the continuation of its repressive policies will have tragic consequences."

Lugar, Kassebaum and Senate Majority Leader Robert J. Dole (R-Kan.) warned Reagan Monday that unless he moved forcefully to pressure the Pretoria government, Congress would enact new sanctions. In a White House meeting, the three made a series of proposals and advised Reagan against flatly opposing sanctions in yesterday's speech. Their advice was not taken.

Democrats condemned Reagan's reiteration of his policy toward the South African government.

House Budget Committee Chairman William H. Gray III (D-Pa.), delivering the official response for the party, said the president had "declared the United States and Great Britain the co-guarantors of apartheid."

"In the eyes of that country's black majority, our nation is firmly aligned with the most oppressive system on earth," Gray said. "It is time to stop thinking about South Africa's minerals and diamonds. It is time to start practicing American values."

Senate Minority Leader Robert C. Byrd (D-W.Va.) accused the White House of "an abdication of American responsibility for political and moral leadership," while Sen. Edward M. Kennedy (D-Mass.) said Reagan's policy "makes America the last, best friend of apartheid."

Visibly disappointed by Reagan's speech, Lugar told reporters he hoped his committee could act on sanctions legislation before Congress begins a mid-August recess. But he warned that the Senate calendar is clogged with divisive foreign policy issues and urged an agreement to prevent filibusters that could endanger a sanctions bill.

The agreement Lugar is suggesting would include a promise of votes on South African sanctions and on continued U.S. compliance with the SALT II arms control treaty with the Soviet Union, which Democratic liberals are seeking, in return for a Democratic pledge not to filibuster military aid to the Nicaraguan rebels, known as contras.

However, such an agreement may be difficult to achieve. Kennedy, a leading advocate of sanctions, said he was "not prepared to trade apartheid for a failed policy in Central America. Apartheid is wrong; contra aid is wrong."

Lugar sought to put an optimistic interpretation on the president's speech, saying Reagan had accepted his advice not to use the term "constructive engagement."

"That is an advance," he said.

But the mild-mannered Lugar also said that while he had hoped Reagan would deliver "an extraordinary message to the world," the president instead had adopted "the most extreme language" of British Prime Minister Margaret Thatcher.

Asserting that Reagan had shown "a lack of leadership," Lugar said: "The president needs to do more. I think the president would like to do more. This may not be the last word."

Reagan's speech came a few hours after the Foreign Relations Committee began three days of hearings on U.S. policy toward South Africa. In an opening statement, Lugar urged a cautious approach and rejection of the House-passed legislation that would impose an almost total trade embargo and require U.S. companies to withdraw their investments.

But Lugar also signaled support for limited sanctions. "These are not actions that close down mines or cause unemployment," he told reporters. "They hit at the heart of the white leadership group."

Senate Democrats continued to urge adoption of the House measure. Kennedy warned that he would attempt to attach the House bill to legislation raising the national debt ceiling if there is no agreement to enact sanctions legislation before the August recess.