One by one, the black leaders of southern Africa -- Marxists, socialists and conservatives alike -- are either acquiescing to peace pacts with South Africa, their giant white-ruled neighbor, or muting their criticism of its apartheid system of racial segregation. There is, however, one vocal exception.

Zimbabwe's prime minister, Robert Mugabe, has emerged in recent months as the region's last angry man, a role that has made him Africa's leading antiapartheid spokesman but isolated him diplomatically. His scathing criticism has brought him into increasing conflict with Washington in addition to Pretoria.

In recent weeks, Mugabe has called for international economic sanctions against South Africa and asked western nations to provide arms to black nationalists seeking its overthrow. He has accused the Reagan administration of instigating South African military aggression in the region and political represssion at home, and has warned that if American officials continue to support Pretoria, "then they have chosen to be opponents of the rest of Africa."

Western diplomat see Mugabe's position as rigid, unyielding and potentially harmful to Zimbabwe's interests, and during the last 18 months America's chief Africa envoy, Chester Crocker, has pointedly avoided Zimbabwe during his regional shuttle missions.

His supporters say Mugabe's refusal to march to the South African or American drumbeat is based in large part on his conclusion that the South Africans cannot be trusted to honor the commitments they have made to countries such as Mozambique and Angola. It is also grounded in his own hatred of apartheid.

Yet, at the same time Mugabe is denouncing Pretoria, senior civil servants in his Ministry of Trade and Commerce are negotiating a new preferential trade agreement with their South African counterparts.

The dichotomy is not unusual for Zimbabwe's austere and enigmatic Marxist leader, a man of impressive intellect who chooses to walk a narrow political and ideological line but who often confounds friend and foe alike by deviating widely from it when necessity demands.

In many ways he is an apt symbol of the country he leads, for Zimbabwe is a frail but ambitious society that struggles to survive while dreaming of leading Africa out of poverty to a revived glory. History and circumstances have forced the Zimbabweans into an uneasy relationship with their hostile neighbor and into an uncomfortable dependence on western capital, but their leaders speak the rhetoric of Marx and dream of socialism.

In the case of South Africa, Mugabe offers verbal encouragement to black rebels while consistently denying them military or material support or a haven inside his territory. He forbids his Cabinet ministers to meet their Pretoria counterparts yet allows a South African trade mission to operate in his capital. Even his call for economic sanctions is tempered by the fact that South Africa is his nation's largest trading parner and its ports and railways the conduits for most of Zimbabwe's exports.

Mugabe's hatred of apartheid is said to date back to the late 1940s, when the Jesuit-trained rural schoolteacher won a scholarship to attend the all-black Fort Hare University in South Africa's Cape Province. There he observed the development of an apartheid system still in its infancy and studied Marx and Gandhi at a school that became the training ground for many of the region's present black leaders.

"After Fort Hare there was a radical change in my views," Mugabe has told biographers David Smith and Colin Simpson.

South Africa gave strong military and moral support to the white-minority government of Ian Smith during its seven-year civil war with black liberation movements under Mugabe and rival guerrilla leader Joshua Nkomo. But when Mugabe won a sizable electoral victory in 1980 and became prime minister of the new Zimbabwe, he offered Pretoria a policy of live and let live.

South African black nationalists who expected to operate a guerrilla movement from inside Zimbabwe's borders were disappointed, and while Mugabe swore never to grant diplomatic recognition to Pretoria, he allowed operatives of his Central Intelligence Organization to maintain close contact with their opposite numbers in South Africa to defuse potential confrontations.

Zimbabwean officials now contend that the South Africans reneged on their end of the bargain. Three South African saboteurs allegedly blew up half of Zimbabwe's Air Force in 1982 and there is strong evidence that South African military officers have armed and trained dissidents who have waged a campaign of intimidation and economic sabotage in the southern Matabeleland region. The officials also hold South Africa responsible for last year's sabotage of a vital oil pipeline in neighboring Mozambique.

For their part, the South Africans contend that Mugabe's relentlessly hostile rhetoric is poor payment for the preferential trade status and occasional technical asistance Zimbabwe receives. "Quite frankly, we're a little tired of being treated like a pariah by Mr. Mugabe," said a South African diplomat.

For a time, the Reagan administration found itself caught between the two countries and quietly helped break through several impasses. But in recent months, Mugabe has increasingly characterized American diplomats as Pretoria's allies and co-conspirators.

The most recent clash came last month following the latest visit by Crocker, assistant secretary of state for African affairs, to the region. The visit was a continuation of his effort to break the deadlock over independence for Namibia and the withdrawal of Cuban troops from neighboring Angola.

Crocker and the South Africans have contended that these issues are related -- that a South African pullout from Namibia can only be achieved alongside an agreement to pull the Cubans out of Angola. Virtually all black African leaders publicly have denounced this "linkage," but some, such as Zambia's Kenneth Kaunda, have continued to play a role in mediating a Namibian and Angolan settlement.

Mugabe, on the other hand, has branded linkage as "blackmail" and shown no interest in seeking a compromise acceptable to Pretoria.

Crocker again chose to bypass Harare on his most recent trip, which the Zimbabweans viewed as a snub. But what was worse, in their view, was that soon after Crocker's departure from Pretoria, South African leader P. W. Botha issued a thinly veiled threat to Zimbabwe. He warned in a speech that those who refused to normalize relations with Pretoria "will soon realize they have chosen an impossible path. If they continue to give preference to a hostile and conflictual relationshiop, their peoples are the ones who stand to suffer most in the end."

The speech deeply angered Mugabe, who responded a few days later in Arusha, Tanzania, by suggesting that Crocker had given at least tacit approval to Botha's get-tough tactics. Mugabe then asked whether the Reagan administration's "constructive engagement" policy toward South Africa had turned into "constructive instigation."

"What one finds rather strange is not just the fact that we are being blackmailed into forging relations with apartheid . . . but the glaring sequence of events that Piet Botha is talking with increased arrogance soon after his meeting with Dr. Crocker," Mugabe said.

American diplomats were unhappy with the timing of the Botha speech, which State Department officials called "unfortunate," but were more upset by Mugabe's strong response. They were particularly unhappy that Mugabe had made no effort to seek a private explanation from U.S. Ambassador David C. Miller Jr. before publicly assailing the United States.

The annual U.S. aid allocation to Zimbabwe was slashed from $75 million to $40 million last year after a diplomatic flap over Zimbabwean votes in the United Nations Security Council. There is some fear here that the allocaton could be cut further this year.

American and South African policy makers had hoped Mugabe's anti-South African stance would ease after the conclusion of his party congress in August. "We thought the rhetoric would ease once he got the radicals out of town," said one analyst. "We sure were wrong. If anything, he's hardened."

The lesson, said another diplomat, is that Mugabe, while flexible when necessity demands, is not the total pragmatist Washington had at one time come to believe he was. "Mugabe's willing to compromise," he said, "but only when he feels he has no other choice."