President Jose Napoleon Duarte and his rebel opponents have assessed yesterday's historic peace talks as a success, but statements made last night and today by each side indicated that their positions on how to end the five-year-old civil war remain far apart.

Spokesmen for each delegation said today that neither of the two most divisive issues between them had come up during the talks in the northern town of La Palma. One was the insistence by the guerrillas of the Farabundo Marti National Liberation Front and its political ally, the Democratic Revolutionary Front, that they be allowed initially to share power in the government without participating in elections. The other was Duarte's insistence that no serious negotiations can take place until the guerrillas lay down their arms.

Most of the public statements made immediately after the talks were positive. Both sides interpreted the decision made at the meeting, to set up a joint committee to hold a round of further peace talks beginning next month, as an important step toward ending the war.

But at least on the guerrilla side, spokesmen took pains today to make it clear that they would stick to their main demands.

Tonight one rebel commander, saying he spoke for the whole general command of the guerrilla front, issued a communique from his mountain hideout raising anew a version of the rebels' demand for sharing power in government.

Ferman Cienfuegos, the commander of the National Resistance, one of the five factions making up the guerrilla alliance and a member of the rebel delegation at yesterday's talks, called for the formation of a "national forum" representing all political and social forces in the country to replace the current Legislative Assembly.

Although Cienfuegos did not specify the exact functions that the national forum would perform, it seemed clear that it was envisaged as a new vehicle for the form of provisional power sharing that the guerrillas have long demanded as a condition for ending the five-year-old civil war.

Guillermo Ungo, the leader of the rebel delegation at the talks in La Palma, said in a telephone interview from his home in Panama earlier today that the issue of the creation of a national forum had been raised at the talks.

The rebel statements differed sharply from the explanation of the talks given by President Duarte and other members of his delegation who had insisted that the rebels had not made any real demands but had only presented their "analysis" of the political situation in El Salvador and said how they wanted it changed.

In a rambling press conference at his official residence tonight, Duarte reiterated that he had made it clear to the rebels yesterday that there could be no negotiated solution that contradicted the country's one-year-old constitution which provides for elections as the only means of political change.

In the telephone interview from Panama, Ungo said that although there was agreement on the necessity of finding a way to end the war, the method of doing it was far from being resolved.

Ungo said that despite the moderate, conciliatory tone adopted by his delegation at La Palma, the guerrilla front had not softened its demands, especially on the sensistive issue of power sharing.

"Our thesis is still that there can be no real settlement unless there is a sharing of power to guarantee its implementation," he said.

"Power sharing remains one of our fundamental goals," he added, "but we are realists and realized that at this stage of the talks to have put such a demand on the table would have risked future dialogue.

"For us this meeting was just a beginning, a first step," he said. "We felt this step had to be taken cautiously, realistically, if it were to guarantee any future dialogue."

Another official of the rebels' political arm, Jorge Villacorta, said from his home in San Jose, Costa Rica, today that the La Palma meeting was a victory for his movement because it had succeeded in institutionalizing the dialogue that he said his movement had been seeking ever since 1981.

"We did not think Duarte was sincere in his talk about dialogue and felt it was all just a political maneuver," Villacorte said. "So what we wanted to do was to turn this maneuver into serious negotiations and that we could only do by avoiding placing all our cards on the table immediately."

In effect, that seems to have been precisely what the Duarte government sought to do by seeming, at least temporarily, to soften its previously hard line on the issue of not holding talks with the rebels until they laid down their arms and agreed to live by the political rules laid down in El Salvador's year-old constitution.

Hardly had the talks ended than Duarte appeared on a television interview broadcast nationwide in which he said he was buoyed by the fact that the establishment of the commission was the key proposal he had made to the four-man rebel delegation during their 5 1/2 hours of talks.

He said that although there remained fundamental differences on the nature of the political realities in El Salvador today, "there was a convergence of views on the fact that the war should end and that there was a national consensus for peace." Meeting with journalists at his presidential residence late last night, Duarte said he "did not mention" the laying down of arms during the meeting. He said the issue had been shelved for the moment "and we will have to see where the road leads."