A conference held here last week, designed to foster good U.S.-African relations, turned instead into a forum for denouncing the Reagan administration's policy of "constructive engagement" with white-ruled South Africa.

That was the dominant mood in a week of discussions among about 200 prominent Americans and Africans at the annual conference of the African-American Institute. Speaker after speaker lashed out at the United States for its policy of linking the independence of Namibia to withdrawal of Cuban troops from Angola, its improved diplomatic treatment of Pretoria and its reluctance to condemn South African aggression and destabilization against neighboring states.

They drew a picture of the United States allied with South Africa against hundreds of millions of black Africans as the continent moves inexorably toward racial confrontation.

The problem, many delegates said, is that while the American diplomatic effort is the main hope for a peaceful solution in Namibia and for bringing pressure on South Africa for internal change, they despair of the United States using its muscle.

Zimbabwe's Prime Minister Robert Mugabe launched a broad-scale attack on U.S. policy in his opening speech. He called the linkage of Cuban troops to Namibian independence "blackmail."

In a plaintive tone, Zambian Foreign Minister Lameck Goma wound up the five-day meeting by saying, "We see the Americans wanting to join a team of three countries including Israel and South Africa in the world that believe in intransigence and discounting world opinion. Is that what America wants?"

Much of the criticism of U.S. policy came from Americans, many of whom were Reagan opponents. The sponsoring organization, the African-American Institute, is a private organization that receives funds from the U.S. government, foundations and corporations to promote better relations.

Rep. Howard Wolpe (D-Mich.), chairman of the Africa subcommittee of the House Foreign Affairs Committee, told reporters, "The African leadership is far more disappointed, far more cynical about U.S. policy than a year ago when they were willing to wait and see" on Reagan's friendly embrace of South Africa.

"Nothing of substance has been achieved in the last two years," Wolpe said, calling on the administration to reassess the policy.

"No one believes us because of our actions supportive of South Africa," he said. "We have indicated very clearly to the South Africans that they can do anything they want in the area without reprimand from the United States."

Most of the criticism centered on the U.S. and South African insistence that there can be no independence for Namibia, which South Africa controls as the territory of South-West Africa in defiance of the United Nations, without a Cuban pullout from neighboring Angola.

The United States, not South Africa, raised the Cuban issue, Goma maintained, and, as many other delegates, he said there was no connection between Namibian independence and the Cuban presence at Angola's invitation.

"The Reagan administration is making the South Africans do the dirty work for them. It is a domestic issue for the Reagan administration," he said, meaning that the president is seen here as being more interested in gaining a political victory at home by removing the Cubans than in ending white-minority rule in Namibia.

Chester A. Crocker, assistant secretary of state for African affairs, gave a spirited defense of the U.S. policy of linkage between a Namibian settlement and Angola.

"We have never asked for any African government to stand up and endorse the doctrine of linkage," he said. "It matters to us not at all whether linkage is adopted or condemned by Africa in general . This is not a Gallup Poll on linkage." Without it, he added, "there is no way to get" a settlement because the presence of 20,000 to 30,000 Cuban troops in Angola is a major concern to South Africa.

He said the matter of Cuban troop withdrawal was up to Angola and added that many African nations were more understanding of the U.S. position in private diplomacy than in their public rhetoric, where "every government has to speak to specific constituencies."

Zimbabwean Foreign Minister Witness Mangwende contested that remark, saying, "I want to disabuse the idea that we say different things about linkage in private than in public."

Robert Keeley, the U.S. ambassador to Zimbabwe, also defended the lack of American pressure on South Africa, saying, "The degree of U.S. influence on South Africa is greatly exaggerated."

He told a Nigerian critic, "It is easy to ask a government to put pressure on another government but difficult to allow pressure on yourself. I wonder how you would feel if we exerted pressure through trade on Nigeria over basic matters such as your constitution."

Many opponents noted, however, that the United States is pressuring Angola over the Cubans at the same time that it has granted numerous concessions to South Africa, such as increasing diplomatic representation and relaxing U.S. restrictions on sale of military and police equipment, technology and nuclear material.

Zambian Foreign Minister Goma asked, "We wonder why the United States and its allies are not taking as strong a stand against South Africa as against Poland. Can it be because a majority of the oppressed people in South Africa are black?"

"South Africa is the rogue in our region," Goma said. He noted that Pretoria often sent its troops into Angola while at the same time demanding withdrawal of the Cuban defenders.

"It is as if South Africa is begging the Angolans to keep the Cubans so they can prevent the independence of Namibia," Goma concluded.

A Nigerian diplomat told a reporter, "The Reagan administration opened a Pandora's box with the issue of linkage. It's up to it to close the box before its term ends."

The strongest criticism of the United States came from Sam Nujoma, president of the South-West Africa People's Organization (SWAPO), which has fought a 16-year guerrilla war for Namibian independence.

He described the linkage policy as "cynical, shameful and inhuman" since it provides South Africa with an excuse to remain in Namibia.

Contrary to Crocker's continued optimism over a settlement, which he once predicted would be concluded by last fall, Nujoma said prospects "remain bleak. The United States is to be blamed."

Shaking his finger at Crocker, the guerrilla leader said, "You are promoting racial hatred in Africa."