HACIENDA NUEVA TRINIDAD, EL SALVADOR -- Leftist rebels, carrying out a vow to step up their campaign of economic sabotage, reportedly killed seven civilians while attacking a large coffee plantation guarded by the Salvadoran military, soldiers and civilians said here.

The attack last Saturday on this farm in central Usulutan province was one of the largest by the insurgents in recent months. It left one of the highest tolls on the civilian population by either side this year.

Four soldiers of the 6th Brigade also were killed, two civilians were wounded and five buildings were destroyed by grenades and fire. Rebel casualties were not known.

According to soldiers who witnessed the attack and civilians who had talked to other witnesses, several hundred rebels attacked from three points at about 11 p.m., using grenade launchers, machine guns and M16 rifles.

Maria Guevara, 39; her daughter Pastora, 20; and Manuel de Jesus Argueta were killed at point-blank range by the rebels when the insurgents burst into the house where they were sleeping, the sources said.

On the blood-stained walls and floor bits of flesh and hair were still visible yesterday.

Jose Sandoval, 56, was shot in a nearby house, and Joel Cruz, 19, was shot as he tried to run out of the building, which had been set on fire, according to the sources. The inside of the building was charred, and burned hammocks and blood-splattered clothes added credibility to the account.

Two cooks, Ede de la Paz Mejia and Ana Deysi, their ages unknown, reportedly were shot and burned in another building.

Violence also has increased in the capital, San Salvador. On Thursday, suspected rebels ambushed a pickup truck carrying four members of the Treasury Police in daylight at a busy intersection near the center of town.

Three died instantly and the fourth, a woman officer, was shot six times and is hospitalized. No one was arrested and no one has claimed responsibility for the action.

Partly in response to the increased attacks, President Jose Napoleon Duarte announced today he would not honor an appeal by Catholic Archbishop Arturo Rivera y Damas for a Christmas cease-fire.

Speaking after a ceremony at the Gerardo Barrios Military School in the capital, Duarte said he saw "no benefit" in a cease-fire because "during past truces the guerrillas have continued their attacks."

Gen. Adolfo Blandon, the military chief of staff, said a truce "made no sense" because the military had discovered guerrilla plans to step up their attacks in the capital before the end of the year, aimed at thwarting the legislative elections scheduled for March 1988.

Earlier this month, the rebels announced a new military campaign called "Everyone eats or no one eats," designed to force wealthy land owners to pay higher wages.

One civilian supervisor on Hacienda Nueva Trinidad said it was attacked because the owner refused to pay a guerrilla-imposed "war tax." Not much coffee was destroyed because most had been hauled out earlier in the day, he said.

The owner, Gregorio Zelaya, does not live on the farm and could not be reached for comment.

According to knowledgeable businessmen, rebels have been demanding a $500 to $1,000 "tax" to spare farms from attack, and many of the wealthy are paying, in effect helping to finance the war.

The farm, 80 miles east of the capital, is perched on the slopes of a volcano, accessible only by foot or four-wheel-drive vehicle.

"We are afraid, and with good reason, because we run a risk working here," said an older man whose brother was killed in the attack. "But what can we do? If we do not work, we have no money."

Civilians asked that their names not be used, saying they were afraid the guerrillas might come back and kill them.

No civilians who survived the attack have returned to work, so first-hand accounts were only available from soldiers who fought the guerrillas. Workers who ventured back after having fled, including some relatives of those killed, confirmed key elements of the story.

Of the 700 workers hired for the coffee harvest, only 201 have worked since the attack, slowing the work and causing fear that some of the harvest may be lost, according to the supervisor.

The toll was not higher because most of the workers live several hours' walk from the farm, and go home on weekends.

For many workers, in a country where unemployment hovers around 50 percent, the cash brought in during the coffee harvest is all they will get until the next harvest.

"We come because we live close by, but those who live far will not come back, they are too scared," one said. "I do not blame them, they are right to be scared."

The U.S. Embassy estimates that the Marxist-led rebels have caused about $1.5 billion in economic losses in the eight-year-old civil war.