Prime Minister Robert Mugabe today sharply attacked U.S. policy in southern Africa, describing as "blackmail" the "American and South African demand" that Cuban troops be withdrawn from Angola as a condition for giving independence to Namibia.

Speaking to an audience composed mainly of U.S. officials, including Chester A. Crocker, assistant secretary of state for African affairs, key congressmen and business leaders, Mugabe said that "the United States' insistence on this linkage has given solace to the South African regime" that controls Namibia.

"That Pretoria is now more daring and aggressive than before cannot be doubted," Mugabe said.

"The United States has obviously introduced a stumbling block that may well impede the decolonization process of Namibia, albeit temporarily," Mugabe said.

The speech to the opening session of the annual conference of the African-American Institute was the sharpest criticism of Reagan administration policy in Africa by this key nonaligned, socialist-leaning country, which is the third largest recipient of U.S. aid in black Africa.

Questioned by reporters about Mugabe's attack, Crocker said, "Everyone has to speak to his own constituency. I'm sure Mr. Mugabe wishes us well and will be the first to congratulate us if we succeed" in bringing independence to Namibia. South Africa has controlled the territory under the name South-West Africa since World War I in defiance of U.N. and World Court demands for its independence.

Crocker said he continued to remain optimistic about a settlement despite Angolan refusal to budge so far on linking removal of the Cuban troops to withdrawal of South African forces in neighboring Namibia.

However, Rep. Howard Wolpe (D-Mich.), the chairman of the Africa subcommittee of the House Foreign Affairs Committee, said Mugabe's speech "reflects the growing loss of credibility in the American effort to secure a settlement of the Namibian conflict."

The United States has been leading an effort by five western nations to bring about a peaceful solution to the 16-year-old war between the South African government and guerrillas of the South-West Africa People's Organization (SWAPO).

"The United States is increasingly viewed as allied with South Africa in the regional conflict," Wolpe said. "I think that perception of an American alliance with South Africa is a formula for disaster for the United States."

Agreeing with Mugabe's claim that white-ruled South Africa is destabilizing its black neighbors, Wolpe said, "South African activities are in direct conflict with American policies to develop these countries. U.S. assistance is being eaten away by South African activities."

The United States is scheduled to provide $225 million in aid to Zimbabwe over the 1980-1983 period.

Mugabe also criticized the United States for:

Failing to demand that South African troops withdraw from Angola, which they have periodically invaded from neighboring Namibia for more than two years.

Its "history of connections" with guerrillas, known as the National Union for the Total Independence of Angola (UNITA), seeking to overthrow the government in Luanda.

Concentrating on trade with South Africa among the non-oil-producing countries of the continent.

Mugabe also asked the conference of the African-American Institute, a private organization financed by the U.S. government and U.S. foundations and businesses that seeks to promote better understanding, to examine the "increasing tendency to give aid with political strings attached" to make the recipients "puppets of the donor countries."

Mugabe's critical remarks toward the United States were in sharp contrast to the warm welcome he has given Chinese Premier Zhao Ziyang, who is on a three-day visit here.

In a speech last night Mugabe called for extensive cooperation between the two countries and cited China for giving the most support to his guerrilla movement during the independence struggle in the country then known as Rhodesia.

Mugabe's speech was delayed for an hour by talks he held this morning with Zhao.

African nations have become increasingly critical of the United States as the Namibian negotiations have become stalemated.

Mugabe's speech contrasted sharply with his remarks after President Reagan took office in 1981. Then he was hopeful that Reagan would be able to bring about more constructive changes in southern Africa than former president Jimmy Carter, who is credited with swinging the United States away from support of white-minority governments.