President Jose Napoleon Duarte publicly ordered Defense Minister Gen. Carlos Vides Casanova today to help guarantee the security of left-wing rebel civilian leaders when they fly here Monday for planned peace talks.

The order was delivered just after Vides Casanova gave a speech declaring that the Armed Forces supports Duarte's offer to meet with the rebels as long as any steps toward a negotiated settlement are within "constitutional norms."

In Salvadoran political shorthand, respect for "constitutional norms" means that the guerrillas cannot be granted a share of political power unless they win it under elections as provided in the constitution. The rebels have rejected this condition in the past, arguing that they could not run candidates in elections because they would be vulnerable to attacks by right-wing vigilante groups.

The president's order clearly was designed to suggest first that he could guarantee the left's security, and also that he controls the Army and not the other way around. He is this country's first elected civilian president in more than 50 years.

"Take note of what I'm going to say," Duarte told a small group of journalists in an impromptu news conference with Vides Casanova sitting at his side.

"I am going to ask the Red Cross to make arrangements for their the rebels' reception, lodging, transportation and security. And I'm going to order -- and I am ordering, in front of you -- the minister of defense to take measures to help the Red Cross in all of its tasks."

In the past, influential conservative factions in the Army have resisted calls for a negotiated settlement. Vides Casanova said that the Army has the capacity to defend the country "with the force of reason" as well as with arms, and he offered to accompany the president into the meeting with guerrilla leaders who have been battling the Army for about five years.

But the defense minister also made clear that the Armed Forces would drop its support of Duarte if he made too many concessions to the left.

"The Armed Forces . . . supports the steps taken by the president of the republic and commander in chief of the Armed Forces, aimed at obtaining peace for our people, with the assurance that they will be framed in the constitutional norms effective in our country," Vides Casanova said at the nation's military academy.

Duarte's guarantee of security to left-wing civilian rebel leaders marked a significant concession. In his original offer to meet the left, delivered at the United Nations on Monday, he said that he was inviting only rebel military leaders because the civilians "lack authority."

The rebel alliance said that it was sending two military leaders and two civilians, and civilian official Ruben Zamora said today in a telephone interview that the civilian representatives would be himself and Guillermo Ungo. Ungo is president of the Democratic Revolutionary Front, the civilian wing of the rebel alliance, while Zamora is a member of the alliance's political-diplomatic commission.

Until now, however, it has been unclear how the two civilians were to enter the country. The guerrillas' military leaders easily can walk to the site of the talks in the northern town of La Palma because it lies on the edge of territory normally controlled by the guerrillas.

Zamora said today that he and Ungo would fly Monday morning to San Salvador's international airport, and that they would be accompanied by four or five non-Salvadoran civilians to guarantee their security. When the left proposed last year to hold peace talks in San Salvador, government officials expressed doubts that the security of rebel negotiators could be guaranteed.

But while saying he and Ungo were proceeding on the assumption that they would attend, Zamora expressed concern over whether logistical details could be worked out during the next two days.

Both sides made modestly threatening gestures today. The Army occupied La Palma this morning, although Duarte had said earlier this week that troops had been ordered to keep away from that area. Salvadoran television showed troops leaving the town, and the commander said that he had withdrawn.

The guerrillas' clandestine Radio Venceremos, meanwhile, announced plans to disrupt transportation nationwide on Oct. 16 and claimed responsibility for a relatively rare bombing in the capital Wednesday.