The Reagan administration yesterday made public what it called "definitive evidence" that the Soviet Union and its communist allies -- principally Fidel Castro's Cuba -- have been acting as tutors and arms suppliers to the leftist guerrillas seeking to over-throw the U.S.-supported government in El Salvador.

Release of the evidence marked the latest step in the administration's escalating campaign to use El Salvador communist support for leftist insurgency movements in the Third World. Over the weekend, the administration warned that it might resert to direct action against Cuba if the flow of arms to the guerrillas isn't halted.

Informed sources said yesterday that Thomas O. Enders, a career foreign service officer currently serving as ambassador to the European Economic Community, has been selected by President Reagan and Secretary of State Alexander M. Haig jr. as the new assistant secretary of state for inter-American affairs -- a post that will make him the operating boss of the administration's high-priority efforts in Central America.

Enders has no previous experience in Latin American affairs. However, in the early 1970s he served as deputy chief of the U.S. Embassy in Phnom Penh and played a key role in the Vietnam war effort to halt the supply of arms to the Vietcong through Cambodia. The sources said his appointment to the Latin American job has the approval of Sen. Jesse Helms (R-N.C.), the conservative chairman of the Senate subcommittee on hemispheric affairs and a strong advocate of a tough anticommunist stance in the region.

The evidence, drawn from captured guerrilla documents and intelligence reports, contained no big surprises, since most of its main points have become public through press leaks in recent days.

Essentially it depicts what appears to be an attempt by communist and radical leftist countries around the world to collaborate in providing large-scale arms assistance to the Salvadoran ensurgents, with most of the weapons and material moving first through Cuba and then Nicaragua.

A narrative summary prepared by the State Department and entitled "Communist Interference in El Salvador" states that sympathetic regimes on four continents pledged to supply the guerrillas with "nearly 800 tons of the most modern weapons and equipment" and that nearly 200 tons of these arms were smuggled into El Salvador for the unsuccessful "final offensive" attempted by the insurgents last month.

In an effort to whip up international support for its stance, the administration last week sent high-level teams to present its evidence to West European and Latin American governments. The first public comment in Washington came yesterday from French Foreign Minister Jean Francois-Poncet, who is here for talks with Haig.

However, his remarks to reporters following their meeting were much more restrained and hedged than U.S. officials reportedly would have liked. Francois-Poncet said the evidence appeared to indicate "external interference" in El Salvador and added that France has "always condemned external interference."

According to the evidence, the guerrillas' biggest arms suppliers appeared to be Vietnam and Ethiopia, with other countries, including the Soviet Union and various of its East European allies, collaborating in a campaign "to cover their involvement by providing mostly arms of western manufacture." The evidence also describes a donation to the guerrillas of $500,000 from Iraq and a promise from Palestine Liberation Organization leader Yasser Arafat to provide weapons and training.

However, the biggest role is ascribed to Cuba. The State Department's narrative says that in late 1979 and early 1980 Cuba played "the direct tutelary role" in bringing the previously feuding Salvadoran guerrilla factions into a united front, subsequently provided the guerrillas with assistance and advice in planning their military operations and eventually became the major agent in collecting the arms and passing them to the insurgents through Nicaragua.

Although the evidence does not establish clearly a direct link to the Soviet Union, the documents describe how Shafik Handal, leader of the El Salvador Communist Party, allegedly visited Moscow last summer during an around-the-world trip to solicit help that also took him Vietnam, East Germany, Czechoslovakia, Bulgaria, Hungary and Ethiopia.

In Moscow, Handal reportedly met with Mikhail Kudachin, a central committee official dealing with Latin America, who suggested that he visit Vietnam since the Vietnamese had large amounts of American weapons captured during the Vietnam war. The documents quote Handel as saying the Soviets paid for his trip to Hanoi.

At a briefing on the evidence yesterday, John A. Bushnell, acting assistant secretary for Latin America, cited that point as an indication that the Soviets gave "coordinating assistance" to Handal's mission. Noting that Handal visited Moscow before and after his trips to Vietnam and Ethiopia, Bushnell said, "It appears that his friends in Moscow were making calls in advance on his behalf."

The State Department narrative said that by last September substantial quantities of the arms promised Handal were in Cuba and being transferred to Nicaragua, whose leftist government contains Marxist elements sympathetic to the Salvadoran guerrillas. In the captured documents, Cuba is referred to by the code name "Esmeralda" and Nicaragua by "Lago."

The narrative said that, after a pause brought about by U.S. protests, shipments from Cuba to Nicaragua resumed in October and November and by December had reached a volume sufficient for the guerrillas to begin planning their offensive. According to the narrative, the arms moved from Nicaragua to El Salvador by air, by sea on small launches and overland through an adjoining border area of Honduras.

As evidence of these shipments, the narrative cited a truck loaded with arms that was seized in Honduras after leaving Nicaragua and two planes from Nicaragua that were disabled in El Salvador. In one case, the pilot was captured and admitted being an employe of the Nicaraguan national airline who had flown earlier arms delivery missions.

The United States has been putting heavy pressure on Nicaragua to cut off the deliveries, using as a spur the threat of ending the U.S. aid that the country needs to stay afloat financially. Bushnell, noting that all U.S. aid to Nicaragua is currently suspended, also confirmed reports that there has been "a lull" in the arms flow from Nicaragua during the last couple of weeks, but added that it is too early to tell whether the movement of equipment has ended.