Europe's last empire collapsed a decade ago when Portugal's "angry young captains," weary of long colonial wars, toppled the rightist regime in Lisbon and pitched five troubled African nations headlong into independence.

Before the "Revolution of the Carnations" swept Angola, Cape Verde, Guinea-Bissau, Mozambique and Sao Tome and Principe to freedom, Portugal's pretensions to a world role were impressed on schoolchildren in books that showed silhouettes of its colonies overshadowing a map of Europe.

Today, Europe's poorest country is turning its pioneering traditions to discover a new African vocation among former dominions still beset by conflict and poverty.

"Portugal is casting itself as a bridge-builder able to draw on the insight gained from five centuries of colonial history to help bring the developing countries of tropical Africa closer to the West," said Leston Bandeira, a journalist based in Portuguese-speaking Africa before and after independence who now coedits a Lisbon newspaper dedicated to its affairs.

In the debate raging here on Portugal's future role in Africa, Bandeira is on the side of the skeptics who fear the country has little left to contribute through its present policies.

Behind the arguments lies a shared feeling that Portugal could offer major trading partners such as the United States and the European Community, which it joins next year, something beyond a domestic market of 10 million people. A characteristic editorial in the influential newspaper Expresso urges: "We must take advantage of having possessed a vast empire, of knowing like the palm of our hand the territory, the culture, the subsoil of some of the richest regions in Africa."

But many observers say the slow healing of colonial wounds has made it too late for Portugal to establish itself as a privileged diplomatic and trade link between its rich allies and developing Africa. The latest trauma straining relations is a row over semiclandestine spokesmen for Angolan and Mozambican rebel groups operating unchecked in the Portuguese capital.

Prime Minister Mario Soares has told African leaders that his Socialist-led government cannot infringe on the rebels' right to free speech and has cited Portugal's lack of an adequate secret service to investigate alleged criminal conspiracies. Although Lisbon has joined the United States in considering the supply of "nonlethal" military aid to Mozambique, it otherwise maintains a policy of strict noninterference in the internal affairs of Portuguese-speaking Africa.

"Portugal will never win back the confidence of its former colonies until it takes sides in the conflicts their governments are facing," said Bandeira. "Viewed from Africa, it is difficult to reconcile Portugal's expressions of support for Angola and Mozambique with the rebel propaganda that originates in Lisbon."

For one Angolan official, who asked not to be identified, Portugal's ambition to become a valued mediator between Africa and the West "doesn't really have much to offer except to Portugal."

Nevertheless, Portugal's first official performance in this role will begin shortly, when Portuguese agronomists, paid through a $1.5 million U.S. grant, arrive in Guinea-Bissau to help the West African nation develop export crops of pineapples and sugar as alternatives to its dependence on cashews.

"This is the United States' first experiment with three-way cooperation -- that is, using our funds to tap the know-how and resources of a third nation better suited to help a particular developing country," said Donald Finberg, for whom the post of trilateral aid coordinator was created last December at the U.S. Agency for International Development.

AID plans to work with Portugal on a $7 million project to grow pepper in the islands of Sao Tome and Principe and is studying three-way enterprises in Mozambique.

U.S. envoys to southern Africa say Lisbon can help the Reagan administration shape its aid to win a positive response from the region's Marxist governments, which have shown frustration with the expense and style of Soviet Bloc assistance.

"I believe most African governments now realize that it is the West, and not the Soviet Bloc, which is relevant to Africa's problems . . . because the Soviet Union and its allies have focused heavily on military and security assistance," said Frank Wisner, deputy assistant secretary of state for African affairs at a Lisbon conference to promote trilateral cooperation.

"Right now Mozambique needs hoes, not tractors," said Prakash Ratilal, governor of the drought-stricken country's central bank. "But in their own interest some countries insist on sending tractors instead of the spare parts, shovels and hoes that we want."

Administrations patterned on the Portuguese model and similarly rudimentary technology placed Portugal in a position to ease the clash of cultures, said AID coordinator Finberg after a recent trip to Mozambique.

Increasing aid to southern Africa to add momentum to the swing away from reliance on Soviet support is a key part of the Reagan administration's policy of "constructive engagement" with South Africa. U.S. officials are involved in efforts to negotiate a withdrawal of 25,000 Cuban troops from Angola linked to an independence settlement for South African-controlled Namibia. The stated objective is peace between white-ruled South Africa and its black neighbors.

Here, too, Portuguese officials say their African affinities can smooth the path of quiet diplomacy by briefing Washington and other allies on the moods and trends of the region.

U.S. Ambassador Allen Holmes, who left Lisbon on June 26 to take up his new post as the State Department's director of politico-military affairs, said the mediation of Portuguese officials three years ago "opened the door" for secret contacts between Angola and the United States that subsequently led to regular negotiations and a marked improvement in relations.

Much of President Reagan's talks with Portuguese leaders during a trip to Lisbon May 8-10 focused on developments in southern Africa, according to aides. Portuguese President Antonio Ramalho Eanes, who visited Mozambique at that time, told Reagan that Marxist African governments were more open to strengthening relations with the West than Nicaragua is, a State Department official said.

Eanes, who fought eight years in Portugal's colonial wars, champions a nonaligned role for Portuguese-speaking Africa. "The effectiveness of our actions in Africa will be to prevent East-West conflict from dominating the continent," he said.