African leaders wound up their annual summit yesterday after issuing a special warning to the United States that any breach of economic sanctions imposed by the United Nations on Rhodesia would be regarded as "an unfriendly act" and "a particular affront to the dignity and aspirations" of the African people.

Passing what was termed an "urgent resolution" in view of the vote expected in Congress this week on Rhodesian sanctions, the Organization of African Unity said that a decision to lift them would constitute a "serious impediment" to efforts to bring about a negotiated settlement of the worsening war in Rhodesia.

The summit was expected to devote much of its attention this year to the growing Cuban and French military involvement in the conflicts of the continent. But no concrete measures were taken to limit it as both "moderate" and "progressive" African states agreed on the right of any nation to call upon outside military assistance to help settle its internal problems.

The Rhodesian sanctions issue suddenly came before the organization's 15th annual summit after African diplomats at the United Nations sent word here that a group of U.S. senators were seeking to lift the sanctions imposed on Rhodesia following the establishment of the new multiracial transitional government there in March.

The Senate group, led by Jesse A. Helms (R-N.C.), is reportedly gathering strong support for an amendment to the foreign security assistance bill. The amendment would suspend American observance of the U.N. sanctions for the rest of this year.

The amendment appears to have drawn enough support to move the Carter administration, which opposes ending sanctions, to arrange a National Security Council briefing last week for 100 Senate aides on the possible harm the amendment could do to American credibility in the Third World.

Bishop Abel Muzorewa, one of the black nationalist leaders of the new government in Salisbury, has been campaigning in the United States for the past 10 days for American support and a lifting of the sanctions that were imposed shortly after the white minority in Rhodesia declared its unilateral independence from Britain in November 1965.

Despite intensive diplomatic efforts by the three African leaders of the transitional government to gain recognition here, the African organization condemned it as "yet another maneuver of the illegal racist minority [white] regime to maintain its domination and oppression of the African people" in Rhodesia.

A delegation representing Rev. Ndabaningi Sithole, another of the transitional government's black leaders, was refused permission to address even the meeting of African foreign ministers prior to the summit and was deported from the Sudan.

The special resolution passed here Friday reaffirmed the African organization's "unequivocal commitment" to the nationalist guerrilla war now being waged by the Patriotic Front guerilla alliance against the transitional government "until complete, genuine freedom and independence" is achieved.

This year's summit of African leaders, the most heavily attended since the organization's founding in 1963, came to a weary close yesterday afternoon after a final 17-hour session devoted alternately to passing some 250 resolutions and fights over three inter-African feuds that continue to preoccupy the continent's politics.

As expected, the summit condemned any pan-African military force initiated or organized by anyone other than the African organization and condemned the existence of all foreign military bases in Africa and "pacts with extra-African powers."

The only country specifically mentioned in this regard was France, whose bases on several small islands off the African coast in the Indian Ocean were condemned as a "blatant aggression" against Africa.

But it also recognizes the "inalienable right" of any African state to take "any measures" deemed necessary to safeguard its independence and territory, thus leaving African leaders free to call upon nations outside the continent for military assistance.

In this manner, the controversial issue of the presence of more than 45,000 Cuban troops and military advisers and at least 8,500 French ones in 20 different African countries was circumvented without any real effort to stem the general malaise at the growing involvement of outside powers in Africa.

Even a Senegalese resolution that would have called upon the Eastern and Western nations participating in the Helsinki Conference to "refrain" from military intervention in Africa was defeated.

The 34 African chiefs of state and heads of government did, however, call for a common African strategy for strengthening the objectives and principles of the nonalingned movement that among other things excludes alliances with any of the superpowers.

By far the most controversial issue at the conference was the fate of the mineral-rich former Spanish Sahara, a territory divided up by Spain in early 1976 between Morocco and Mauritania and where an Algerian-backed liberation movement is fighting to set up an independent state.

After a stormy all-night session Friday, the summit passed a compromise resolution setting up a committee of five heads of state under the chairmanship of the Sudanese president, Jaafar Nimeri, to study "all the facts" in the Western Sahara dispute, including the exercise of the people's right to self-determination in preparation for a special summit to discuss the issue. A similar heads-of-state conference on the former Spanish colony failed to take place last year.

In this manner, the African organization averted once again a confrontation between its "moderate" and "progressive" members over an issue that risked splitting it seriously.

On other bilateral disputes, like the Ethiopian-Somali and Chad-Libyan ones, the organization made little headway in its mediation efforts, and urged its special committees set up to deal with them to continue efforts to find a peaceful solution.

For the first time, an African leader, President Nimeri, raised the question of the 17-year-long war in Eritrea, saying it had become a threat to the peace of the entire continent and was not just an "internal problem," as Ethiopia claims. And the summit called upon the committee charged with mediating disagreements between Sudan and Ethiopia to examine the "root causes of the problem," an indirect reference to Eritrea.

Other resolutions passed at the conference attacked Iran for its hostile attitude toward African efforts to get the oil-producing states to stop supplying South Africa and asserted the organization's determination to apply an oil embargo against the white-ruled nation.

Moderates won the struggle for the designation of the body's new secretary general on the sixth ballot, with the election of Togolese Foreign Minister Edem Kodjo, and generally showed a surprising strength and aggressiveness in the debates here this year.