After grueling and often acrimonious discussions between rebel and government representatives, Salvadoran President Jose Napoleon Duarte went on national television tonight and bluntly rejected as unconstitutional a tough, three-phase rebel peace proposal.

The guerrilla proposal envisioned the formation of a new transitional government, changes in El Salvador's year-old constitution and a drastic reorganization of its armed forces.

"I can never move from my respect for the constitution," Duarte said in the broadcast. His chief negotiator, Minister to the Presidency Julio Adolfo Rey Prendes, said the rebel plan "violated every law in the constitution."

The only significant positive development to emerge from the second round of talks today was a limited Christmas truce for the nation's roads and highways. Reading a joint communique from the back of a flatbed truck parked near the Salesian seminary where the talks took place, Archbishop Arturo Rivera y Damas said the proposal he made for Christmas season "tranquility" on the nation's road had been accepted by both sides.

Little else seems to have transpired, and although the two sides agreed to continue the dialogue, the timing for their next meeting significantly was not stated, as it had been after their historic encounter in the town of La Palma Oct. 15.

The joint communique said that aside from agreeing to the limited truce on the roads and that further meetings should be held, the two sides agreed on procedures for future meetings of their joint dialogue commission and exchanged points of view on their widely divergent positions.

The highway truce was far short of even a temporary cease-fire and provided only that civilian vehicles would not be shot at or attacked from Dec. 22 to Jan. 3.

The surprisingly hard-line guerrilla statement, which reiterated positions considered unacceptable by the government, demanded the expulsion of U.S. military advisers, a freeze on all foreign arms shipments and a cease-fire after territory controlled by the rebels and the government was delineated. A bilateral commission of the guerrillas and the Army then would be set up to guarantee its implementation.

The rebel proposal seemed to raise virtually every issue that the Salvadoran armed forces have said was not negotiable, from power sharing to integration of the rebel forces into a new national Army.

The uncompromising tone of the rebel statement contrasted sharply with recent public statements made by a number of their political leaders that hinted at a willingness to consider some form of participation in next spring's parliamentary and municipal elections as Duarte had invited them to do.

"They are asking us to replace our president and the constitution," complained Jose Antonio Morales Ehrlich, secretary general of Duarte's Christian Democratic Party. "We can't do that."

Morales Ehrlich said that he hoped the proposal was just one the guerrillas were putting on the table "for later concessions" and added that the government intended to keep on talking despite the unacceptable tone of the rebel demands.

The proposal of the guerrillas' Farabundo Marti National Liberation Front and its political arm, the Democratic Revolutionary Front, was presented to government representatives this morning at the start of the second round of peace talks at the seminary here nine miles south of the capital of San Salvador.

As about 1,500 persons gathered on the highway below the seminary to demonstrate for peace before a cordon of heavily armed National Police officers, the proposal was presented to the government negotiating team in the presence of Rivera y Damas, who has been acting as mediator.

Foreign analysts here speculated that the tenor of the guerrilla statement might reflect a bargaining ploy to set the outside limits of the negotiations. A more troubling possibility, these analysts said, was that the statement reflected the view of the hard-line guerrillas fighting in the hills, rather than the more moderate political leaders who represent them abroad. If that was the case, then the sides seemed to be poles apart, and the prospects for success in the talks would be slight.

Only a week ago, Hector Oqueli, a member of the rebel delegation here, told reporters in Mexico City that it would not be politic for the guerrillas to raise their previous demands for power-sharing at this time. Yet the document he came here to present to the government did just that in no uncertain terms.

The tough guerrilla proposal, presented by four Marti Front representatives led by Ruben Zamora, a leading Democratic Revolutionary Front official, was the guerrillas' reply to Duarte's proposal at La Palma that the guerrillas lay down their arms and participate in the country's new democratic system.

While the rebels' proposal did not directly address itself to Duarte's offer, it rejected the main premise: that conditions in El Salvador had changed dramatically since the rebels took up arms against the state five years ago.

"The causes that pushed our fronts to fight using political and military means, are still in effect," the statement said. "The death squads have not disappeared, nor the illegal arrests, nor the tortures; they have only become more sophisticated."

Duarte originally had appealed to the guerrillas by insisting that two national elections that had resulted in the establishment of a constituent assembly, a new constitution and his own presidency were signs that El Salvador was moving toward democracy and that the rebels could compete politically without fearing for their lives.

The tough rebel proposal did not break up the talks, which lasted about 12 hours.

Sources close to the talks said the conversations had been prolonged because the participants were discussing procedural issues for further talks.

Despite Duarte's rejection of the guerrilla proposal presented today, it still may cause serious debate within the government and armed forces in the days to come.

In the first phase of negotiations as presented in the rebel proposal, a national forum would be organized where all sectors of Salvadoran society and politics could make their views known.

They also suggested accords on human rights and political liberties that would dismantle death squads and other repressive institutions, end torture and the disappearances of people and bring those responsible for past violations of human rights to justice.

In this first phase the rebels proposed, too, that steps to "humanize the war" be taken, such as agreeing to respect the Geneva conventions and halting the bombing of civilians.

Also to be negotiated in this phase was the question of getting rid of U.S. military advisers and the freezing of arms imports by both sides.

The one concession the rebels seem to have offered the government in this phase is an accord to halt economic sabotage.

The second phase of discussions, the rebel proposal suggested, would be an accord on broadening the government to include representatives "of all political and social forces" in the country, in other words power-sharing with the rebels.

During this phase of discussions the issue of a cease-fire, with delineation of territory between both sides, also would be negotiated, as would be the resettlement of refugees.

The final phase of negotiations as envisaged by the rebels today would involve the installation of a new, broad-based government of "national consensus," the changes in the constitution to take into account the agreed-upon political changes, the reorganization of the armed forces and -- only after all these things had taken place -- new national elections.

While politicians such as Duarte, who did not attend today's talks, might be prepared to view the rebel demands as part of the negotiating game, the rebel proposal seemed certain to irritate the armed forces, which hold great power here.

Ever since Duarte first proposed the dialogue in early October, there has been great nervousness about the talks within the officer corps.

While Duarte repeatedly has consulted with the officer corps about the talks and won their grudging acceptance to date, he has only been able to do so by pledging that he would refuse to negotiate on the issues raised by the rebels today.