El Salvador's Christian Democratic Party and the armed forces boycotted the formal inaugural session of the newly elected constituent assembly today, saying the ruling military-civilian junta had not been officially invited by an "arrogant" right-wing assembly leadership.

No high-ranking military officer attended because of the absence of junta member Gen. Jaime Abdul Gutierrez, chief of the armed forces.

The boycott was a political act evidently designed by the Christian Democrats to embarrass the newly elected chief of the assembly, former Army Maj. Roberto D'Aubuisson, whose rightist supporters control the legislative body. He delivered his welcoming speech to a half-empty hall, nevertheless telling the assembled diplomats, including U.S. Ambassador Deane Hinton, that the day opens "one of the most beautiful chapters of our national history."

The inaugural session of the assembly came before formal agreement was reached on the appointment of a provisional president to rule the country until a presidential election can be organized. The assembly is expected to name banker Alvaro Alfredo Magana over the objection of D'Aubuisson's party.

Although they boycotted the inaugural session, the Christian Democrats returned for the afternoon working session of the constituent assembly.

Members of D'Aubuisson's Nationalist Republican Alliance, known as ARENA, its Spanish abbreviation, were furious, insisting that all members of the 30-month-old military-civilian junta, including the Christian Democratic junta president Jose Napoleon Duarte, had been invited properly, receiving formal invitation cards signed by D'Aubuisson.

"It means they talk a lot about democracy, but they don't believe in it," said Chamber of Commerce President Conrado Lopez Andreu.

The junta, installed by a military coup in October 1979, officially runs the country until the installation of the new president of the provisional government. The elections March 28 named 60 deputies empowered to select the president, outline his duties and write a new constitution and organize presidential elections sometime in the next year or two.

Several D'Aubuisson supporters said they were concerned at the absence of armed forces leaders at today's session, but dismissed the Christian Democrats' boycott as what one called a childish protest.

Christian Democratic assembly leader Julio Rey Prendes said at a press conference after the session that there had been no invitations, "either because there was a little bit of disorder, or by bad intention." In either case, he said, "We thought it was not proper to participate when the junta was not invited." The four-member junta, he said, had organized the March 28 elections, kept them clean and "merits the respect of the whole nation, and, if I may say so, the world" for having done that.

The Army, he said, could not participate when its chief did not. The only military men present handled crowd control and the flag display.

Rey Prendes admitted that the boycott was also a political protest against what he called "the great arrogance" of the two rightist parties that dominate the assembly. The three parties have been negotiating fruitlessly since the election to form a joint government, with the Christian Democrats insisting on a share of power equal to their 40 percent of the valid votes, and the other two groups arguing that their 60 percent share entitles them to run the country.

Asked if the rightists' anger might not rupture the ongoing talks, Rey Prendes said that was possible.

Party officials said there was general agreement that Magana, 56, would be named over the protests of ARENA, the result of a shotgun marriage between the Christian Democrats and the party of the traditional landowning elite here, the National Conciliation Party.

The armed forces, prodded by U.S. congressmen and a visit from special U.S. envoy Vernon Walters, suggested strongly to the parties that electing an independent nonpolitical president like Magana would be the best way to satisfy Congress that economic and military aid here should continue.

Under that kind of pressure, National Conciliation leaders reluctantly agreed to have their 14 deputies join the Christian Democrats on this one issue, and then moved to help ARENA exclude the Christian Democrats from any posts of power in the assembly. Party.

The armed forces, prodded by U.S. congressmen and a visit from special U.S. envoy Vernon Walters, suggested strongly to the parties that electing an independent nonpolitical president like Magana would be the best way to satisfy Congress that economic and military aid here should continue.

Under that kind of pressure, National Conciliation leaders reluctantly agreed to have their 14 deputies join the Christian Democrats on this one issue, and then moved to help ARENA exclude the Christian Democrats from any posts of power in the assembly.