Nicaragua's guerrilla-backed junta tonight called for a joint armed force including members of the National Guard and Sandinista guerrillas and promised safe passage out of the country for military and government officials "not found involved in crimes against the people."

The statement, released in Costa Rica, was the most conciliatory made by the prospective junta government to date, and follows two days of intensive negotiation with special U.S. Ambassador William Bowdler.

Entitled "Plan of the National Reconstruction Government to Achieve Peace," the statement called for the immediate resignation of President Anastasio Somoza to the Nicaraguan Congress, and for the Congress to turn power over to the junta.

The junta would then dissolve both Congress and the Nicaraguan constitution, which Somoza partisans rewrote in 1974.

The junta would then order a cease-fire, with both Sandinista and National Guard troops maintaining their positions and, under orders from the junta, safeguarding public order.

Those guardsmen who respect the cease-fire and junta authority "can integrate themselves into the new army or civilian life as they desire," the statement said.

The junta is scheduled to resume talks with Bowdler tomorrow morning. Earlier today, junta members said the talks were characterized by a "more cordial U.S. attitude" than in previous stalemated negotiations.

While they said the United States had requested a news blackout while talks continued, the junta's leaders expressed cautious optimism that an accord could be reached on a future Nicaraguan government and that there was agreement on "mutual fundamental concerns."

These concerns include Somoza's resignation, the orderly transition of power to a junta-based administration and the structure of a government that will be able to "gain the confidence" of Nicaraguans and Western Hemisphere nations.

While the United States has agreed to provide for Somoza's resignation, discussions center on the second two points. U.S. officials believe that they are best guaranteed by adding more moderate members to the junta and preserving Somoza's National Guard as the foundation of any future Nicaraguan army. Tonight's junta statement addreses the latter point.

In a separate news conference in Costa Rica, junta representatives today released a lengthy economic and social outline for their government. The plan includes the development of a mixed economy, with some sectors - including finance and foreign commerce - to be run by the state or as joint state-private enterprises, and others to remain the exclusive province of the private sector.

Although the plan was not very specific, the junta in the past has announced its intention to nationalize Somoza's extensive business and land holdings and some other privately held agricultural land that is uncultivated.

More than 40 Nicaraguan landholders, businessmen and academics have worked on developing the program in Costa Rica, including former officials of the Central American Common Market and Nicaraguans on leave from international banking institutions.

While their politics very widely, all have pronounced themselves against Somoza. Many have donated their services out of a desire to protect their own interests from possible radical programs of the Sandinista National Liberation Front guerrillas or to make an early contribution to the new government.

In one of the few substantive changes to previously announced plans, the junta broadened a proposed state council that will perform legislative functions from 30 to 33 members. In addition to political and business groups, the council will now include representatives from the Nicaraguan Association of Professionals, the National University and the clergy.

While preparations for a new government continued, however, Somoza today gave a morale-boosting speech to his Liberal Party. He blamed the "destruction, looting, burning and assassinations" in Nicaragua on "communism encouraged from abroad," and spoke of a "diabolical conspiracy against the people of Nicaragua to abolish our public freedoms and destroy our democratic institutions."

The United States is deeply concerned that failure to arrive at a political agreement with the junta will mean a Sandinista-dominated government. At the same time, Sandinista military advances in the continuing civil war have increased rebel confidence and increasingly demoralized the National Guard.

A massive National Guard counter attack launched against rebel-occupied Masaya, 20 miles south of here, has made little headway against an estimated 3,000 Sandinistas in the city.