Forty-eight hours before elections and on the eve of a rebel-declared truce, Salvadoran guerrillas launched a mortar attack on a police academy west of the capital, authorities said Friday.

Rebels launched the 20-minute attack shortly after midnight Thursday, wounding two soldiers, destroying two vehicles and causing "serious damage" to buildings, the military said.Farabundo Marti National Liberation Front reb-els have declared a three-day electoral truce beginning today and have promised not to disrupt elections for the first time in more than a decade of civil war that has left an estimated 75,000 people dead.

About 250 international observers are in the country to supervise the voting.

In other fighting, four rebels and four civilians were wounded in skirmishes in the northern province of Chalatenango and the northeastern province of Morazan, the military's press office said.

Military sources said a national army operation called "Plan Democracy" that began last weekend has encountered little resistance from the FMLN.

But an international observer in El Salvador for Sunday's congressional and municipal elections said the situation was "very tense" in the guerrilla stronghold of northern Morazan. Troops from an elite, U.S.-trained battalion and two regionally based military detachments have moved in to the area.

"It's an ugly situation and it's going to get worse," said the observer, who asked not to be named.

Dismissing the FMLN's unilateral truce as "propaganda," the armed forces put "Plan Democracy" into effect with the stated goal of "guaranteeing that the Salvadoran people can exercise their right to vote in a climate of tranquility on March 10."

In Sunday's elections, voters will choose mayors, as well as all 84 members of the single-chamber national legislature.