President Anastasio Somoza today called a U.S. proposal that he surrender power to an interim government an "attempt to overthrow" him. He declined to say whether he would accept the proposal until it comes to a vote in the Organization of American States.

In a communique issued in Costa Rica, the Sandinista National Liberation Front flatly rejected any outside solution to the Nicaraguan crisis "with or without Somoza."

The Sandinistas called on Nicaraguans "not to abandon your arms" and rejected "all forms of intervention" in their civil war against the Somoza government.

In an emergency OAS meeting Thursday, U.S. Secretary of State Cyrus Vance called for a government of "national reconciliation," presumably without Somoza, and recommended the OAS provide peacekeeping troops to maintain order during a transitional period.

Although the Sandinistas had turned down categorically any negotiated solution even before Thursday's meeting in Washington, Somoza is clearly hoping that the resolution Vance presented, as well as a counter resolution presented by 13 Latin American nations, will never receive the required two-thirds majority in the OAS.

In a telephone interview today, Somoza indicated he was depending on a split among his enemies to prevent passage of a substitute resolution. OAS member countries opposed to Somoza have been divided between those who want to intervene to stop the war and those who think Nicaraguans should fight it out among themselves.

The later pro-Sandinista group, including Mexico, Panama and Jamaica, "think they can win the war," Somoza said. He has accused Panama of directly aiding the rebels with personnel and arms.

Somoza repeated his assertion that the Sandinistas represent a Moscow-backed Communist conspiracy against his government. He expressed dismay that the rest of the hemisphere would not come to his aid when a Sandinista force crossed Nicaragua's southern border from Costa Rica.

"Listen, I'm man enough to take my medicine when the time comes," Somoza said, "but I cannot accept the actions of anyone who doesn't recognize certain realities."

International law binding allies to come to each other's aid when one suffers armed aggression, Somoza said, "is just a piece of rubber."

Asked about the contention that, regardless of who aids the Sandinistas, there will be no peace in Nicaragua until he leaves, Somoza said, "I could say there'll be no peace in Nicaragua if I don't get a square deal." That deal includes, he said, the retention of and sharing in power of his own Liberal Party and the National Guard.

Somoza said that Vance's proposal was basically the same one made to him last September, when the United States began and eventually aborted efforts to mediate an earlier outbreak of civil war by recommending that he leave office.

Political analysts here, including members of both the moderate opposition and Somoza's government, said they believe the war had gone too far for an effective negotiated solution.

"If a peacekeeping force comes here," one high level official said, "it will just end up fighting a war against the Sandinistas."

At the same time, although the National Guard takes its orders directly from Somoza, longstanding Sandinista vows to extend "the people's justice" to governmeit soldiers and officials found guilty of crimes against the populace has hardened the Guard's desire for victory.

Despite the apparent faltering of their principal offensive in the south, where government troops reportedly still hold guerrilla forces within several miles of the border, the Sandinistas clearly feel they have the strong support of the Nicaraguan people. As diplomatic support for their cause grows, they have become more determined to beat Somoza militarily.

Last weekend, Venezuela, Ecuador, Boliva, Colombia and Peru, the five members of the Andean Pact, recognized the Sandinistas as a legitimate army eligible for international assistance.

Mexico, Costa Rica, Ecuador and Panama have broken relations with the Somoza government and Panama Thursday formally recognized a newly named five-member Sandinista provisional government.

The Carter administration fears Sandinista control over the government, and a possible hard swing to the left, and wants its membership expanded to include what Vance called "all important sectors" of Nicaragua. Administration sources said Washington wants both Somoza's Liberal Party and the National Guard represented in any transitional government.

While two members of the Sandinista provisional junta are moderate political figures, the other three are a Sandinista Front leader, a leftist intellectual and a Socialist union leader.

The Sandinistas recently have softened their tone on the post-Somoza National Guard, agreeing to give favorable consideration to those soliders guilty only of following ordera and not of torture and murder. The guerrillas have insisted repeatedly, however, that no part of the government Liberal Party or the National Guard will be included in any post-Somoza leadership.

[However, the Sandinistas said in a communique released early Saturday morning that they executed six men, including the head of Nicaraguan customs, by firing squad following a trial in a revolutionary court, Reuter reported.]

On the military front today, Somoza said the National Guard was advancing in its attempt to rout guerrillas from a zone of several square miles in eastern Managua and that he expected the international airport access road, closed by fighting for nearly two weeks, to be opened shortly.

He said that once government troops control Managua, they would head for Leon, the country's second largest city 54 miles north of here, to dislodge Sandinistas who now completely control it. In the northern cities of Chinandega and Matagalpa, Somoza said the Guard was preparing counterattacks against rebel-held zones.

In the cities of Esteli, Rivas, Masaya, and Diriamba, Somoza said the government was "holding our own" against the guerrillas.

Meanwhile, approximately 60 foreign journalists remaining in the Intercontinental Hotel checked out this morning when they awoke to find the entire staff had left.

The hotel, which the Sandinistas have designated a "military target," in recent weeks has turned into a virtual barracks for government and military officials and their families and bodyguards seeking safety from the war-torn streets.

Last week, approximately 30 hotel employers remaining of a normal contingent of 300 to serve the hotel's more than 200 rooms expressed a desire to close out of fear of large numbers of soldiers and armed government sympathizers on the premises.

The journalists, with few alternatives other than the Intercontinental, central Managua's only large hotel, persuaded the reduced staff to stay on their jobs. But the hotel employes vowed they would leave immediately if any of them was arrested. Thursday, the swimming pool attendant was dragged off to National Guard headquarters across the street after he allegedly spoke insultingly of the National Guard and was overhead by soldiers.

Although the man was later released, his temporary arrest - in addition to the Sandinista threat of an attack on government officials in the hotel and a fear that hotel employes would be considered government collaborators in the event of a Sandinista victory - was the last straw for the staff.

Slipping singly and in small groups from the premises before dawn today, they left guests to awaken alone.

By midmorning, however, police had arrived with acting manager Ramon Ortega, who said he had decided, unlike many other employes, not to hide but simply to go home. The police found him there this morning and brought him back. Ortega said he was not sure if he was under arrest.

While government officials quickly sent for civil servants to man the phones, they said they would keep the hotel open only as a place to sleep and that all other services were suspended. All journalists staying in the hotel decided to leave, with most going to private homes of friends or small residential hotels.

At least a dozen journalists decided to leave the country today, joining 37 who left Thursday for what they said was their own security and a protest against the slaying Wednesday of ABC correspondent Bill Steward by a National Guard soldier.

A military hearing begun Thursday into Stewart's death was at least temporarily suspended today when a National Guard captain wanted for testimony could not be released from combat duty in eastern Managua's slum neighborhoods. CAPTION: Picture, Sandinistas advanced toward a critical junction held by the Guard near La Virgin. By Sandinista Soldier Emilio via AP