SAN SALVADOR, El Salvador (AP) _ Leftist guerrillas attacked the country's most important army base and an engineer detachment Tuesday, killing eight people and wounding 17.

The attacks came as representatives of El Salvador's major political parties met in Mexico with top commanders of the guerrilla army to discuss a modified insurgent electoral proposal the rebels say could end the war.

''Rejection of the proposal will mean the war will not only continue, but will escalate,'' said the Farabundo Marti National Liberation Front's radio.

The 9-year-old war has killed an estimated 70,000 people, most of them civilians.

The early morning attack on the 4,000-member 1st Brigade's headquarters was the fifth on a major military installation in this capital since November. With jurisdiction over the capital and its environs, the brigade headquarters is considered the country's most important military installation.

The commander of the 1st army brigade reported no casualties inside the downtown base and that damage was negligible, but he refused to allow journalists inside.

The rebels used two pickup trucks fitted with catapults. The detonation of one explosive charge launches the bombs and simultaneously blows up the vehicle.

The explosion of one of the vehicles killed Pedro Martinez, about 70 years old, and his wife Maria Teresa, about 65, who lived in a house near the headquarters.

The rebels also attacked the headquarters of the Detachment of Engineers in Zacatecoluca, 35 miles southeast of San Salvador. Officers there said three soldiers, including a major, were killed at Zacatecoluca; one civilian and a Civil Defense trooper were killed in fighting nearby.

Martinez's body lay on the sidewalk in front of his humble home an hour after the vehicle parked in front of it exploded at 6:20 a.m.

Col. Orlando Zepeda, commander of the first brigade, said Mrs. Martinez, whose legs were blown off by the blast, died on the way to the hospital. He said three people, all civilians were wounded by the explosions.

The explosions broke windows, gouged walls and blew doors off hinges in 12 homes.

In Zacatecoluca, guerrillas and soldiers battled for three hours before dawn around the engineers' base, according to officials there, including base commander Col. Benjamin Canjura.

He said Maj. Donald Sampson was killed in the fighting. Three other soldiers were reported killed when a rocket-propelled grenade hit the truck in which they were traveling. One civilian and one Civil Defense trooper were killed in a clash in a nearby village.

Canjura said 13 soldiers and one civilian were wounded in the fighting. There was no immediate word on guerrilla casualties.

In Mexico, the guerrillas proposed that elections be postponed from March 19 to June and that a cease-fire be insituted. It was a switch from previous demands the election be postponed until September, but left constitutional problems unresolved.

The constitution calls for President Jose Napoleon Duarte to leave office on June 1; right-wing and center political parties have opposed any change in that date and the army has hinted of a coup if Duarte overstays.

The guerrillas say the parties are hiding behind the constitution because they don't want to negotiate seriously.