Harvard University President Derek Bok, who has resisted pressure for a decade to divest the nation's best-endowed school from companies doing business in South Africa, called yesterday for tough economic sanctions against the Pretoria government.

Testifying before the Senate Banking, Housing and Urban Affairs Committee, Bok said current U.S. policy of "constructive engagement" toward South Africa is "increasingly inadequate," adding that its "fruits . . . are meager indeed while the suffering and repression continue undiminished."

The U.S. policy involves trying, through friendship rather than confrontation, to persuade South Africa to drop its apartheid policy of rigid racial segregation.

Bok, who announced no change in Harvard's investment policy, was joined in his call by University of Pennsylvania President Sheldon Hackney and former U.N. ambassador Donald McHenry.

Bok's sharp criticism of the administration policy added a significant voice to the chorus of civil rights groups, church leaders and even conservative Republicans advocating greater U.S. pressure for change in South Africa.

Lawmakers of both parties have said that a bill containing sanctions against South Africa will pass this year and that the debate now deals with how tough a signal to send and how soon to impose sanctions.

With sentiment shifting toward punitive economic measures, the Reagan administration has become increasingly isolated in its defense of constructive engagement.

Its policy has been the object of continuing protests and vigils at the South African Embassy here and on campuses across the nation and of frustration expressed by conservatives about Pretoria's seeming intransigence. A report Thursday that not all South African troops have been withdrawn from Angola has also fueled the protests.

McHenry told the committee in emotional terms that America's close association with South Africa's white minority risks making that country "fertile soil" for communism or the kind of anti-American revolution that occurred in Iran or Nicaragua.

Bok, Hackney and McHenry were testifying in favor of a bill, introduced by Sen. Edward M. Kennedy (D-Mass.) and Lowell P. Weicker Jr. (R-Conn.) and considered the strongest of several antiapartheid proposals.

It would prohibit U.S. loans to the South African government, sharply restrict new investment there and ban U.S. sales of South African gold krugerrands and computers.

Senate Majority Leader Robert J. Dole (R-Kan.) and Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) have proposed milder sanctions, but Lugar said Thursday that he is "ready to consider" tougher sanctions than those in his bill.

During more than a decade of student protests seeking divestment by Harvard, Bok has adhered to a middle course, condemning apartheid and opposing bank loans to Pretoria while trying to play what he termed a "positive role" as an investor in U.S. firms that do a small amount of business there.

Bok has tried to balance pressure for total divestment with the argument that stockholder status gives the university leverage on firms to improve working conditions for South African blacks.

The Kennedy-Weicker bill, Bok said, "offers a way of capturing most of the strengths of both opposing points of view" on a governmental rather than a university level.

"Unlike the more extreme alternatives," he said, the bill "does not rely predominantly on the carrot or the stick to achieve reform. It uses both and in a way that imposes the least possible burden on the innocent bystander."

Meanwhile, a delegation of members of parliament from most of the Western European democracies ended two days of meetings here with members of Congress and other groups advocating sanctions.

Jan Nico Scholten, spokesman for the group and a Christian Democratic member of parliament in the Netherlands, said the group hopes to introduce sanctions bills at home to present a united front of western allies against South Africa and capitalize on increasing public sentiment against apartheid.

"There is something going on in Europe," Scholten said. "There is a new spirit, and we saw the same thing here in Washington."