President Francois Mitterrand today underlined foreign policy views increasingly divergent from those of the Reagan administration in a speech stressing global interdependence and the importance of aid to the Third World as a way to help developed countries pull out of their own economic difficulties.

In a speech opening a United Nations conference organizing aid to the 31 countries designated as the poorest in the world, Mitterrand said rich nations such as the United States and France must increase foreign aid if they want their own economies to prosper.

Although Mitterrand's remarks reiterated previously stated policy views, they followed a period in which France has acted on a number of key issues on which Paris and Washington disagree.

Two weeks ago the French ambassador in Pretoria tried to organize a diplomatic censure of South Africa for its eviction of squatters in Cape Town, a move blocked by U.S. opposition. Last week France joined Mexico in recognizing the leftist alliance fighting a U.S.-supported junta in El Salvador as a "representative political force." Last night Paris voted against Washington in the U.N. Security Council, supporting a resolution to condemn South Africa for its incursion last week into Angola.

Today Mitterrand declared: "Helping the Third World is helping yourself pull out of the economic crisis. Who still dreams of getting developed economies on the move again in a lasting way without the help of new markets, new partners, new worlds to work with, make exchanges with and speak with equal to equal?"

The stress on interdependence, a favorite theme of Mitterrand's, put a spotlight on sometimes clashing viewpoints in the Paris of Mitterrand and the Washington of President Reagan on how Europe and the United States should respond to the Third World's economic needs and political turmoil. In the view of French and American diplomats, including French Foreign Minister Claude Cheysson, this is a likely subject of discord between the two recently installed governments.

The differing approaches on Third World topics contrast with broad agreement on a need to be firm with the Soviet Union in Europe and Afghanistan. Reagan expressed pleasant surprise at the convergence of views on the Soviet Union after his first meeting with Mitterrand in July at the Ottawa summit. This, together with an apparent willingness in the White House to take Mitterrand at his word that the four Communists in his Cabinet pose little danger, has generated a better atmosphere than initially thought possible between the two administrations.

On Third World issues, however, the French and American paths are swiftly heading in opposite directions.

French participation in the declaration with Mexico on the leftist alliance in El Salvador flowed from a conviction of Mitterrand and his advisers that domestic injustice and political strains are more to blame for that country's war than interference by Cuba or the Soviet Union. This is a far different interpretation than that of Reagan and Secretary of State Alexander M. Haig Jr., who repeatedly have accused the Soviet Union and Cuba of trying to extend their influence through subversion.

In South Africa, the French Foreign Ministry had ordered its ambassador in Pretoria to organize a diplomatic protest by Western nations at the way the South African government rousted out black squatters in makeshift suburban shacks. According to a ministry spokesman, "The initiative failed because of a lack of adequate response." News reports from South Africa said the main obstacle was the U.S. Embassy, which refused to go along as part of the administration's policy of avoiding confrontation with South Africa.

Mitterrand's policies reflect a heritage of Socialist sympathy for liberation movements in the Third World. What to the Reagan administration appears to be an effort by the Soviet Union to strengthen its position in Central America tends to appear to Mitterrand's as the uprising of economically deprived people under repressive governments.

The heritage is close to Mitterrand personally and to his advisers. The president's wife Danielle belongs to an organization, Salvador Solidarity, which seeks to alert French public opinion to the sufferings of the Salvadoran people.

Mitterrand picked as a foreign policy adviser on his Elysee Palace staff a theorist of Latin American revolutionaries such as the late Che Guevarra. The aide, Regis Debray, has written widely and admiringly of such movements but since joining Mitterrand's staff has kept out of the public eye.

During a tour last month of Central America, Cheysson referred in one meeting with journalists to "American imperialism." According to published reports here, Mitterrand's aides back home winced at that language, but it did seem to suggest at least part of the atmosphere in which France is forming its Third World policies.

Mitterrand has gone out of his way, however, to keep the tone of relations with Washington friendly. His speech today, for example, flatly contradicted U.S. policy on aid in some points. But its language was moderate and he avoided direct mention of the United States in his insistence that developed nations must help poor countries more than in the past.