Saudi Arabia, in a surprise announcement on the opening day of the 60-nation Afro-Arab summit conference here, has pledged to give $1 billion in new development assistance to the countries of Africa.

The aid, announced tonight by Saudi Foreign Minister Prince Saud Faisal, represents one of the biggest single infusions of money ever given by an oil-rich state to the underdeveloped countries.

It is still less than half the aid the African countries hoped to achieve at this conference and it carries with it no pledge that the impoverished nations of Africa will get the discount on petroleum prices that they want. But it neatly defuses the only potential source of controversy at this summit, an ambitious attempt by 60 Third World countries to find unity of purpose and action.

The $1 billion will be in the form of grants to increase the lending capital of the Sudan-based Arab Bank for Economic Development and the African Development Bank, based in the Ivory Coast.

According to Saudi statistics, Saudi Arabian Foreign aid contributions to "international and regional institutions" have been running about $650 million a year. Thus the new Saudi pledge means that Africa alone will get an infusion of aid that exceeds what the Saudis have been giving yearly throughout the world.

For the Saudis, who have a revenue surplus of more than $1 billion a month, the pledge hardly represent a major commitment. Moreover, the economic plight of the African states is so severe that $1 billion of loan funds is not going to solve their problems.

But Faisal's speech was warmly applauded by African delegates at the summit meeting because it represented a breakthrough that the African states feared they would fail to achieve here.

The summit conference brought together in rhetorical and ceremoniaL if not actual, harmony delegates from countries linked by geography and history but divergent in race, religion, politics and language. Predictably, most of the program consisted of speechmaking.

In the first two sessions delegates were addressed by President Anwar Sadat of Egypt, Felix Malloum of Chad, Idi Amin of Uganda and Leopold Senghor of Senegal. They heard from Yasser Arafat, chairman of the Palestine Liberation Organization, and from representatives of Tunisia, Nigeria and Mauritius.

[Contacts are under way to arrange a meeting between King Hussein of Jordan and Arafat, who last met in 1970, Reuter reported, citing sources close to the summit talks.]

The themes of the speeches were those that have become familiar over the two decades that conferences of nonaligned and Third World countries have been taking place: opposition to imperialism, Zionism, colonialism and racism, and support for endeavors to control the countries' own resources and destinies.

Sadat set the tone, saying that "we are an effective and influential power" as a group, committed to "liquidating imperialism and its stooges" so that a "handful of countries" no longer control the world.

The meeting, in the People Hall of the Arab Socialist Union, a dormant organization that once embodied the politics of Gamal Abdel Nasser, is less than a summit of Arab and African leaders.

Fewer than half the 60 countries are represented by their heads of state. Some of the most renowned leaders of Africa did not come: Jomo Kenyatta of Kenya, Julius Nyerere of Tanzania, Sekou Toure of Guinea, Mobutu Sese Seko of Zaire.

King Khalid of Saudi Arabia is absent, as is Col. Muammar Qaddafi of Libya. The emir of Kuwait, the king of Morocco, the president of Tunisia stayed home.

But all Arab states are represented at some level and only one black African nation, Malawi, stayed out altogether.

The official program calls for adoption of three policy papers adopted last week at a preliminary meeting of foreign ministers. One is a political program supporting national sovereignty, territorial integrity, self-determination, noninterference by one state in the affairs of another - all ideas to which the participants subscribe in theory if not in practice.

The second paper establishes a 10 point program of economic cooperation and the third is to outline a program for achieving the first two.

If there was to be any real controversy at the summit conference, it was expected to be over the economic issue.

At the foreign ministers' meeting before the summit, the Africans backed off their demand for an immediate infusion of at least $2 billion in development aid and accepted instead an Arab pledge to invest more in Africa and to coordinate economy policies.

Besides the $1 billion that Saudi Arabia pledged tonight, the Arabs have also agreed to give $6 million to the Liberation Committee of the Organization of African Unity to support liberation movements fighting the white regimes in Rhodesia and South Africa. A third of that is coming from Saudi Arabia and a sixth from Egypt, although Egypt is a country with no cash to spare.

Sadat said in his speech that one purpose of the conference was "finding the most effective means of deepening the political and economic isolation of Israel, South Africa and Rhodesia."

The tone of the conference, however, is not one of militancy against Israel, with which many of the black African states once had good relations.