Cecilia Juarez tried hastily to arrange some branches over the face of her son Daniel, 25.

His body was lying in a cattle truck with four others, to be carted away for burial. Too stunned to cry, Juarez apparently hoped to give her son some final dignity by covering his shattered features with dusty leaves.

Daniel was one of 20 villagers, including four small children, killed during a four-hour attack by leftist guerrillas Monday night on this hamlet, 35 miles southeast of the capital, San Salvador.

While some of the dead were local militiamen who apparently were the direct targets of the attack, residents said that at least 12 were unarmed civilians.

The attack, and the assassination of some of the villagers, marked a conspicuous return by the guerrillas to the tactic of killing government supporters in villages, and appeared to be part of a new rebel push to expand their military domain south of El Salvador's capital.

The incident brought sharp condemnation of the rebels Wednesday by human rights groups in El Salvador and the United States.

Juarez said uniformed men had appeared at her home Monday night, identifying themselves as government soldiers. They led Daniel away, unarmed, after asking him to take them to the headquarters of the local Civil Defense force, or militia.

"They told us not to be nervous, it was safe to come out because they were from the government," Juarez recalled.

Tuesday morning, just before reporters arrived in the town, she found her son's body lying in a road, shot several times at close range and with his throat slit.

Beside him were four other villagers, who also were lured from their homes by leftist guerrillas posing as Army regulars, captured and executed, family members said.

Reporters, alerted by a routine government announcement of an attack on the village, found a macabre panorama of death.

Bombs that rebels had planted at the doors of one Civil Defense member's house burned to death six civilians who were hiding inside, including a 67-year-old grandfather and four children, the youngest a 2-year-old girl.

The guerrillas were seen carrying a list of men whom they believed to be members of the Civil Defense, residents said.

In the propaganda they left behind, the estimated 100 rebels identified themselves as a unit of the Popular Liberation Force, the largest of the five armies that make up the guerrilla front.

The rebels accused the Christian Democratic government of President Jose Napoleon Duarte of press-ganging farmers into the Civil Defense "to massacre the civilian population, even if they are women or children."

They demanded that local residents warn them in the future of any Civil Defense activities.

Civil Defense member Abelardo Lopez, 18, said there were more than 150 militia members in Santa Cruz, but only 15 rifles. The guerrillas took seven rifles.

Lopez said a local Army commander organized the Civil Defense in 1979 to be on the lookout for guerrillas, because no regular Army troops are based in the hamlet.

The gunning down of local government officials and Civil Defense commanders were a rebel tactic in 1979 and 1980, the early years of El Salvador's civil war.

Frequently the victims were accused of being government informers and the rebels claimed to be "bringing them to justice."

Murders by the rebels came during a period when government troops were also frequently accused of beating or killing farmers they suspected of having links to the rebels.

In recent years, reports of arbitrary killings by rebel forces of their prisoners or of local government officials have been less frequent, as the guerrillas concentrated their war in the countryside and sought to win the trust of the rural populace.

But by late last year, as the Salvadoran Army grew to 45,000 troops and increased its antiguerrilla sweeps, new reports of rebel murders multiplied.

The attack here at Santa Cruz Loma marked the largest single such incident since the attacks began to escalate.

While the Farabundo Marti front has appeared to be on the defensive across much of El Salvador this year, Santa Cruz residents said the Popular Liberation Force has been quietly building its strength south of the capital, going from village to village scaring away the Civil Defense.

Rebels killed five Civil Defense members in February in a neighboring hamlet, and warned at that time that they would soon move against Santa Cruz.

Forming and training village-based civil patrols nationwide to report guerrilla movements to nearby Army garrisons has been a key aspect of the United States' strategy for crushing the leftist insurgency here.

"The Army was in power in this region until a year ago," said villager Adan Vazquez, 47, whose son Ismael was one of the unarmed victims. "Now it's the guerrillas who cause the Army more casualties when the two sides fight."

In San Salvador, Maria Julia Hernandez, head of the Legal Aid office of the Archdiocese of San Salvador, reported Wednesday that several "grave violations" had occurred in the attack. At least six unarmed prisoners were shot dead, she said, "and that simply is not acceptable."

In the past, the office has tended to concentrate its criticism on government forces, which it has charged with committing the bulk of human rights abuses here.

Americas Watch, an independent U.S. human rights monitor, called on rebel political leaders "to publicly denounce this killing of unarmed civilians."