The 60 members of El Salvador's newly elected constituent assembly took office today in a raucous and emotional ceremony, its shouts and silences reflecting the continuing deadlock among rival political parties trying to divide power and form a new provisional government.

A decision by the two dominant rightist parties to abandon negotiations and form a government excluding the moderate Christian Democrats was apparently reversed at the last moment.

For a nation trying to free itself from a half-century of military-backed rule, however, it is the armed forces that have emerged as the main defender of the reforms instituted following the military overthrow of Carlos Humberto Romero in October 1979.

Military leaders reportedly have warned the right that to keep the Christian Democrats out of the new government would risk the wrath of much of the Army. The defense minister, Gen. Jose Guillermo Garcia, was said to be the first to note that one proposed government slate agreed upon by the rightists contained at least three people who had been cabinet ministers in the Romero government. Their inclusion, he let it be known, would be unacceptable to the Army.

The United States also has played an important role in keeping the talks going, insisting to the increasingly self-confident right that Congress may not be willing to fund an exclusively rightist government.

All sides agree that the talks are at a very delicate stage, with any of the three parties as likely to walk out as to concede crucial points.

"The only solution would be for the Army to define itself very clearly, but they too are divided," said a prominent Christian Democrat, as the legislators took their seats today. "At least things are less tense today."

Members of the rightist National Conciliation Party and the Nationalist Republican Alliance (ARENA) have said privately that they think Congress has a distorted view of their political views and will ultimately support them, even without the Christian Democrats. Acting on that belief, they reportedly circulated secret messages to their 33 assembly members yesterday that the body would convene immediately after the delegates received their formal credentials and would name a new provisional president and vice president, even though the Christian Democrats had refused to agree on names.

Sources said the plan was aborted when ARENA decided to name as president its leader, former Army major Roberto D'Aubuisson, rather than ARENA businessman Jose Rodriguez Porth, considered more moderate, and important members of the National Conciliation Party balked.

If the plan had gone through, "there would have been complete chaos here," said the Christian Democrat. Instead, the delegates and their alternates received their formal documents of office amid only moderate uproar, as ARENA supporters packing the galleries shouted and applauded for all deputies who were not Christian Democrats.

As each Christian Democrat received papers, the modern assembly hall fell silent except for shuffling feet as other Christian Democrats stood up to show respect.

Asked why no Christian Democrat supporters were in the galleries, party secretary Julio Samayoa frowned. "We were told no one would be here but the press and party officials," he said. Applause was predictably loudest for D'Aubuisson, who nevertheless made a point after the ceremony of shaking hands with Christian Democrat delegates.

The three main parties have waged a battle of press conferences during the past week, taking turns denouncing each other as intransigent. Even in private, party officials appear to have hardened their positions.

Asked what the Christian Democrats were offering to tempt the rightists, a high party official said, "We are promising half a million Salvadorans, tranquil, working and listening instead of in the streets shouting." Asked if he was making a threat, he only smiled.

The Christian Democrats received 543,000 votes in the March 28 elections, 35.3 percent of the 1.5 million ballots cast.

But the rightists scoffed at the prospect of rebellious Christian Democrats, noting that the combined vote of the five opposing parties was more than 800,000. Another 188,000 voters abstained or mutilated their ballots, either in protest or ignorance.

"They the Christian Democrats cannot seem to understand that they lost the election," said National Conciliation Party official Dr. Francisco Jose Guerrero. "They will have the power they deserve, but no more." He and many other rightists are convinced that Congress will have no choice but to "respect the verdict of the people," as he put it, by supporting the right.

The Christian Democrats do not trust the right's desire to "perfect" recent Salvadoran reforms, which have put an estimated 60,000 landless peasants to work on large cooperative farms and have opened up bank credit to some peasants. "We want to be in the government with enough power to guarantee the reforms," said Julio Rey Prendes, the party's legislative leader. "If they through arrogance say no, then God save El Salvador."

Asked at a press conference about U.S. involvement in the talks, he said, "We don't believe the influence of any outside government can guarantee the future of the reforms. That we must do ourselves."

According to sources close to the talks, U.S. Ambassador Deane R. Hinton has kept in careful touch with all the negotiators but has tried to avoid specifying any particular form for a government that Congress would find acceptable. He is reportedly telling those who ask his views that the structure of the new government does not matter, so long as the three major parties agree on it.

The 1979 declaration in which the Army announced it was taking power contained strong language calling for sweeping social and economic reforms. But the junior officers who launched the coup soon fell out with more conservative higher officers who objected to the presence of leftists in the governing junta. It was restructured to bring in the Christian Democrats in early 1980.

The Army has remained divided over the range and strength of the reforms, and longtime observers here say that there is always coup talk within one sector or another. It is not Garcia but Vice President Jaime Abdul Gutierrez, an Army general, who is regarded as the chief defender of the reforms and the two men are the poles about which military politics revolves.

But both Gutierrez and Garcia agree that too much work has gone into producing free elections to give up on the civilian politicians just yet. "The people are cynical enough already" about the worth of the political process, said a Central American diplomat. "The Army doesn't want to step in again."

"Time is on the Christian Democrats' side," the minority party leader observed, "because people will soon begin to feel a vacuum of power when they cannot get anything done. It will also remind people that the armed forces are waiting."