President Jose Napoleon Duarte and his Christian Democratic Party today called a mass rally for Friday to underline their public support as the four right-wing parties that have emerged from elections as the dominant political force continued to maneuver for a government that would exclude Duarte.

In his first public appearance since the vote, Duarte arrived at a "victory dance" at his party headquarters this afternoon to announce, amid shouting, tearful supporters, that "the country went out in the streets to vote, the country decided to back the solution that we, the Christian Democrats, have presented."

But he warned, "There remain some who have not understood . . . . They are making errors that can put the stability of the country in danger. We are going to the public."

As the Christian Democrats sought to preserve a measure of power and influence, the final, as yet uncertified, vote tally from Sunday's elections gave the conservative parties 36 seats in the 60-member constituent assembly against the 24 of Duarte's party. The percentage breakdown of the vote remained roughly as it it stood throughout the count, 40.7 percent for the Christian Democrats--a 10-point lead over the second-place party of ex-major Roberto D'Aubuisson and far ahead of the other parties to the right.

D'Aubuisson's party won 19 seats and is contesting two more unofficially allotted to smaller conservative groups. The official party that ran the country's politics for the military and the economic elite from 1961 to 1979, National Conciliation, took 14 seats while the more moderate Democratic Action Party got two and the Salvadoran Popular Party got one, according to the president of the Central Elections Council, Jorge Bustamante.

Of the announced 1,054,291 valid votes, 143,284 were null, blank or the subject of disagreement among the ballot counters.

Under Salvadoran law the constituent assembly, which could be convened within two weeks, will become the supreme authority in the country, with the power to dissolve the current government and appoint a new one by a simple majority of the delegates.

Until then, Duarte and the Christian Democrats will continue to share responsibility in the government with younger military officers who took power in an October 1979 coup aimed at breaking the stranglehold on power by the landed oligarchy and its traditional military allies.

The United States, which is providing military and economic aid vital to El Salvador's survival and which has based much of its Central American policy on the outcome of the struggle in this nation of 4.5 million, has placed its weight behind Duarte and the reformist military officers until now. Reagan administration officials are known to feel they will have a difficult time getting Congress to continue aid to a government that tosses aside Duarte and the reforms he has championed.

The right-wing parties that won seats in the assembly announced yesterday that they had joined together informally as a "new majority" and that they were not inclined to include the Christian Democrats in the new government.

One senior official of the National Conciliation Party said last night that formation of a new junta to replace the four-man executive now headed by Duarte and the vice president, Gen. Jaime Abdul Gutierrez, was under discussion.

An advantage of a junta, rather than a single interim president, would be that prominent party leaders aspiring to the presidency in anticipated 1983 elections would not be barred from running by the Latin tradition of no reelection.

Another option, one official said, would be a junta composed of representatives from the three conservative parties with the most seats and one military man to help smooth the transition to civilian rule.

The exact role of the military in the current political maneuvering remains vague, with no senior officers commenting publicly. But there was considerable talk among the right-wing parties about the possibility of running the defense minister, Gen. Jose Guillermo Garcia--often described as the most powerful man in the country--as a "national unity" candidate for president. When this was published today in the local press, the National Conciliation Party hastily made a public denial that it would field Garcia as a candidate.

Christian Democratic leaders said today in interviews that a new military-civilian junta might be one way they would consider sharing power with the conservative parties in a government of national conciliation.

A senior Christian Democratic official said his party favors a junta in which there would be one Christian Democrat, one military man, and one representative of the right-wing parties. But their public call for the right wing to talk with them has thus far been ignored.

United Press International quoted Christian Democratic Party Secretary General Julio Rey Prendes as charging that D'Aubuisson's party had committed fraud by ousting Christian Democratic observers from polling places in Sonsonate province and altering ballot results. However, Rey Prendes, in making what apparently was the first charge of fraud in this election, added that it did not significantly change the outcome. The Nationalist Republican Alliance had no immediate response.

As Duarte announced the rally, in Liberty Park at 4 p.m. Friday, and an upcoming television address this week, he appeared careful to present this as a contrast to the mass rallies of the past, which frequently became confrontations and often ended in massacres by government forces.

"Go out in the street; we'll be singing songs," Duarte said.

"Five hundred thousand votes cannot be ignored," Duarte told reporters afterward. "I cannot let my people wonder what is happening. I have to be with my people. I have to be with them in the park," he continued. "My duty is to lead, not to wait and see what happens. I have to be in front."

The message of the rally would be clear, Duarte said. "We are the first force in the country."

Meanwhile, the civil war continued unabated in the countryside. There was a second full day of heavy fighting about three miles southeast of the provincial capital of Usulutan, where the main guerrilla force that assaulted the city earlier in the week has been based since last Thursday.

From the highway north of the village of Palo Galan, the crash of artillery shells and mortars could be heard as well as the clatter of .50-caliber machine guns fired from two U.S.-made Huey helicopters .

Refugees from the Palo Galan reported that a force of 600 guerrillas had occupied their town last week and sent about 100 of their troops to attack Usulutan. By election day, they had fought their way to the center of the city, only five blocks from the major Army garrison there.

After four days of fighting in the city, they withdrew under the cover of darkness to Palo Galan, leaving behind a small rear guard of snipers who were active, according to Usulutan residents, until the early hours of today.

The Salvadoran government has rushed reinforcements from its U.S.-trained Atlacatl rapid-reaction force in an effort to smash the guerrilla force.