The Senate, in a decisive defeat for the Reagan administration, voted overwhelmingly last night to impose economic sanctions against South Africa as a protest against the white government's apartheid policy of racial separation.

The 80-to-12 vote reflected growing criticism nationwide that President Reagan's policy of "constructive engagement" with South Africa has left the United States increasingly isolated as Pretoria's chief ally and political defender.

The House has already approved sanctions stiffer than those in the Senate proposal, and a House-Senate conference committee is expected to meet next week to try to resolve differences.

South African Ambassador Herbert Beukes said his government would have no comment until the measure becomes law.

Imposition of sanctions "places the South African government on notice that good relations with the United States require substantial reform," said Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.), who pushed the bill as a compromise approach. He called it "a genuine, positive contribution to promoting an end to apartheid."

Backers acknowledged that the bill's impact would be more symbolic than real and have little immediate effect on South Africa's economy. But they insisted that the symbolism would be important in putting Congress on record in opposition to apartheid, official policy in South Africa since 1948.

"This is a long overdue message of support for South African blacks," said Sen. Claiborne Pell (D-R.I.), who cosponsored the measure. "It makes it very, very clear that the United States is committed to ending apartheid."

The bill would ban new U.S. bank loans and exports of nuclear technology to South Africa and further curb computer sales, while requiring U.S. companies with more than 25 workers there to observe a slightly tightened version of the "Sullivan principles," which mandate desegregation, pay raises and promotions for blacks.

U.S. firms would be required to "take an active role" in opposing apartheid.

Further sanctions, including a ban on new U.S. private investment in South Africa and on importation of krugerrand gold coins, could be considered in 18 months if South Africa is found to have made no progress in ending apartheid. In a new twist, the measure authorizes minting new U.S. gold coins in four weights identical to krugerrand denominations.

The House-passed version bans private investment and krugerrand imports immediately and lifts the bans if apartheid is curbed.

Senate critics unanimously affirmed their opposition to apartheid but argued that sanctions might erode U.S. influence with the South African government and lead to increased restrictions on blacks.

Sanctions also would drive some companies with many black employes out of business, the critics said.

"It is a blight on the United States for us to take this action against a friend that has been an ally in every war," Sen. Barry Goldwater (R-Ariz.) said.

Sen. Nancy Landon Kassebaum (R-Kan.), chairman of the Foreign Relations subcommittee on African affairs, said she would vote for the bill as a compromise measure but warned that it might be "replacing 'constructive engagement' with a form of destructive disengagement with no change in the results."

"Constructive engagement" stresses diplomatic efforts and minimal public rhetoric in pushing negotiations toward ending cross-border violence between South Africa and its neighbors. The administration holds that this would provide Pretoria with enough security to allow domestic reforms and major concessions to its internal critics, especially blacks.

Until the last minute, the vote was threatened by filibuster. The Senate voted, 88 to 8, on Wednesday to restrict Sen. Jesse Helms (R-N.C.), who then let debate proceed in the expectation, aides said, that no strengthening amendments would be attached to Lugar's bill.

"My purpose was to prevent, if I could, loading the Senate bill with the kind of garbage Sen. Edward M. Kennedy D-Mass. and others were promoting," Helms said later.

Kennedy and other liberals, including Sen. Alan Cranston (D-Calif.), agreed to withhold amendments to prevent another filibuster. "You could say Helms was the chief architect of the way it came out," an aide to a liberal senator said.

Several last-minute conservative efforts to weaken the bill were overwhelmed, and the Senate voted, 90 to 2, against substituting the House-passed version.

Helms said that vote would prevent House-Senate conferees from doing anything stronger than the Senate approach. Kennedy later disagreed, saying, "This is a good bill, and I think the bill that will come out of the conference committee will be even stronger."