This small West African nation will be the focus of Africa this week as 48 African national leaders and 3,000 delegates gather here for the 16th annual summit of the Organization of African Unity.

The week-long conference, traditionally closed to the press, has been preceded by rejection of a delegation from Zimbabwe-Rhodesia, expulsion of two rival delegations from Chad and the shelving of a proposed Pan-African peacekeeping force.

On the agenda are such thorny issues as Tanzania's actions in toppling Uganda's Idi Amin, the dispute between Morrocco and Algeria over the Western Sahara, and conflict between North African Arabs as a result of the Egyptian-Israeli peace treaty.

At the center of all the debates will be the incoming chairman of the OAU, Liberia's President William R. Tolbert.

Tolbert has just achieved reconciliation with Liberian dissidents after riots in Monrovia in April, and is respected as a mediator.

The riots, in which 412 Liberians were killed, hundreds injured, and property damage totaled $50 million, were Tolbert's severest test in his seven years in office.

African leaders, however, credit Tolbert with healing the quarrel that for years pitted the conservative countries of the Ivory Coast and Senegal against the radical government of Guinea.

Tolbert has staked much on this first OAU summit meeting in Monrovia. Liberia, despite its limited means, has spent more than $25 million on the conference center and a hotel complex to house the delegates. Tolbert's critics say he is spending too much on this conference, while most of Liberia's 1.5 million people live in poverty.

The OAU conference has been a major effort for Tolbert because he contends that the campaign for a continent-wide African organization began here two decades ago.

In 1959, when other African states were agitating for independence from colonial rule, three of this continent's independent leaders met in the northern Liberian town of Saniquellie to discuss African unity in the years to come. They were the late Liberian president William Tubman, the late Kwame Nkrumah from Ghana and Sekou Toure from Guinea.

Since it formally was founded in Addis Ababa in 1963 the OAU has grown to include 49 independent African nations.

Sovereignty, a sensitive issue in Africa is expected to be a major point of contention between outgoing chairman Jaafar Nimeri, president of Sudan, and Tanzanian President Julius Nyerere.

Nimeri is angry over Nyerere's part in deposing Idi Amin in Uganda. Since Amin's overthrow, some 100,000 Ugandan refugees have flooded southern Sudan.

On other issues, there was unanimous agreement to reject feelers from Zimbabwe-Rhodesia on sending a delegation to the conference. Zimbabwe-Rhodesia's Prime Minister Abel Muzorewa is viewed here as a puppet of his country's white minority.

Delegates were unable to agree on which of two delegations from Chad should be recognized. As a result, neither delegation was admitted.

A plan for a Pan-African peacekeeping force was set aside after delegates became embroiled in questions of sovereignty, ideological differences and how such a force should be financed.