In Angola, a Brazilian supermarket chain runs 25 stores supplying food to a million residents of greater Luanda. In Mauritania, a Brazilian contractor is building an airport and an 850-mile road called "the Highway of Hope." In Nigeria, full facilities for Abuja, the future national capital, are to be installed by the Brazilian firm that built Brasilia.

Blessed by georgraphical nearness and bearing Third World credentials, Brazil is crossing the South Atlantic and moving rapidly into Africa.

Eager to widen trade with an alternative to the superpowers, African nations are welcoming Brazil's overtures, which have resulted in a sixfold increase in Brazilian sales there during the last decade. This year, mutual trade is expected to hit $2.5 billion, boosted in part of generous credit lines from an expanding network of Brazilian banks.

"There is no suspicion that Brazil wants to establish a neocolonial relationship in Africa," Brazilian Foreign Minister Ramiro Elysio Saraiva Guerreiro said in a recent interview, shortly before flying to Nigeria at the head of a Brazilian trade mission.

Political observers here point out that Brazil's initmate ties with the leftist governments of Angola and Mozambique could clash with the recent rightward swing in U.S. African policy.

In its first few months, the Reagan administration has warmed to South Africa, frozen aid to Mozambique and courted Jonas Savimbi's rebels in Angola.

By contrast, during the same period, the Brazilian foreign minister assailed South Africa's apartheid policy of racial segregation as "a crime against humanity" and vehemently denounced South African incursions into Angola and Mozambique.

Last year, accelerating a commercial and diplomatic drive into Africa, Guerreiro led a trade delegation to Mozambique, Angola, Tanzania, Zambia and Zimbabwe, Encouraged by the warm welcomes extended to Guerreiro, Brazilian President Joao Baptista Figueiredo plans to tour West Africa later this year.

Two hundred students from Nigeria, Brazil's largest African trading partner, are studying in Brazil. Last year, the oil-rich African nation was the world's largest importer of Brazilian-made Volkswagens, now sold in 22 African countries.

Agricultural trade between Africa and Brazil remains low because both southern regions grow the same crops. But Africans interviewed here said they are attracted by Brazil's middle-level farming technology.

"U.S. tractors now have everything from air conditioning to televisions," complained Ivory Coast Ambassador Charles Gomis. "The Brazilian tractor is a basic 1945 model -- simpler, sturdier, cheaper -- and our mechanics don't need degrees from MIT to work on them."

Linked by language to Africa's five Portuguese-speaking countries, Brazil is queitly supplanting Portugal as those nations' major trading partner. In 1975, Brazil was the first nation to recognize the new government of the Popular Movement for the Liberation of Angola in Luanda, and the payoff is clear: Last year Brazil exported more to Angola than to neighboring Peru.

Last month, a weekly air link between Luanda and Rio was inaugurated. Next month, the Angolans are expected to open an embassy in this remote capital, joining seven other black African states.

"Jungle capitalist" is a term often used to describe Brazil's economic system, and the Latin nation's political life is controleed by a fervently anticommunist military. Thus, to some observers, it may seem odd to watch Brazilian officials wining and dining a visiting delegations from such Marxist people's republics as Angola, Congo, and Mozambique.

"Markets are markets -- Brazil is pursuing a nonideological foreign policy, which is the only one the country can afford," Riordan Roett, director of the Washington-based Center of Brazilian Studies of Johns Hopkins School of Advanced International Studies, explained recently.

Brazil's evenhanded foreign policy is dictated largely by serious balance of payments problems. The bill for imported oil eats up half its exports earnings, and another quarter goes to servicing the country's massive $56 billion debt -- the world's largest.

Brazil's conservative military is said to also welcome the increasing links to Africa for long-range geopolitical reasons. Virtually all of the Brazilian subcontinent lies to the east of Washington, and many Brazilians believe their nation's manifest destiny is to turn the South Atlantic into a Brazilian lake.

Brazil has the world's second largest black population -- Nigeria's is first -- and racial inequality in Brazil could prove to be an obstacle to closer ties with Africa.

"I lived in Washington in 1960 and what has happened [there] since then is simply extraordinary," said one West African ambassador who asked not to be identified. "Here, they haven't even gotten to the Martin Luther King stage -- the blacks you see are always cleaning up. A delegation of 200 Brazilian businessmen came to my country, and not a black was among them."

Although black history courses are only beginning to be taught in Brazilian schools, one historical footnote has caught the eye of Brazilian diplomats.

At the beginning of this century, about 3,000 black Moslems returned to West Africa in a colonization similar to that of Liberia, which was founded by freed U.S. slaves. Many of their descendants, known today as "Brasiliens," now occupy elite positions in Nigeria, Togo, Ghana and Benin.

On his four-day trip to Nigeria last month, Guerreiro paid a courtesty call to a village where descendants of the retornados live and still dress for commemorative masses in the yellow, green and blue colors of the Brazilian flag.