What appears to be another mass murder of civilians has been discovered in El Salvador's northern province of Chalatenango, where 28 bodies, most of them mutilated, were discovered scattered under a bridge on a river bank yesterday morning.

Farm workers living in the area said that most of the corpses, all but one of men, were dressed "like us," in peasant clothing and that some of the dead had been shot and some were "cut all over." The residents said the bodies were buried yesterday afternoon, under the supervision of government security forces who ordered a mass grave dug without obtaining the necessary permit.

Salvadoran law requires that all unidentified bodies must be examined by a justice of the peace, who must then authorize all burials. By the time reporters arrived on the scene early this morning, the site had been turned into a burial mound, surrounded by stones and marked by two sticks. Small puddles of blood dotted the sand, and the beach was littered with bullet casings of standard government-issue German G3 and Belgian FAL automatic rifles.

In a statement yesterday, the armed forces said they "lament such a horrendous crime directed against humble peasants and condemn this type of violence. The domination of an entire people by terrorism will not be permitted by the armed forces, which reiterate their implacable determination to combat terrorism and subversion."

Salvadoran security forces have been accused of participation in a number of mass slayings of civilians ostensibly suspected of subversion in recent months. In one case, where 30 civilians were pulled from their homes and shot on April 7, their bodies left littering the street of a San Salvador suburb, the U.S. State Department said it believed the government Treasury Police were responsible.

In most instances, however, the Defense Ministry, while acknowledging that some abuses of authority have occurred, has refused comment on, or denied, specific charges and has maintained that troop discipline is improving. The issue of official involvement in acts of terrorism against noncombatant civilians has caused sharp disagreement in the United States, where opponents of the Reagan administration's program of economic and military aid to the ruling junta have called for a suspension of assistance.

Both the Salvadoran and U.S. governments have charged that leftist guerrillas are responsible for the bulk for the hundreds of unexplained noncombatant killings each month in El Salvador. Accounts by residents in Chalatenango, an area that is one of the strongholds of leftist guerrillas seeking to overthrow the junta, left responsibility for the incident in question.

A peasant woman said that she was awakened by shots in the middle of the night. "There were many," she said, "perhaps continuing for half an hour. The earth trembled. Afterward I heard voices on the bridge. When I looked up I aw a small truck leaving. Others among my neighbors saw three trucks in all."

The next morning, she said, she went to the bridge. "All over the river bank there were bodies lying about," several of which she said had their hands tied behind their backs.

The killings took place just after midnight, long past the beginning of a nightly nationwide curfew during which government military forces patrol in vehicles and have authority to shoot curfew violators on sight.

Residents speculated on the identities of the 28 bodies. Before the hasty burial, some peasants arrived to determine if any of the bodies were local residents but, according to one peasant youth, they went away without speaking. "I think they were scared," he said.

Reporters attempting to ascertain if a burial permit had been issued were told by the justice of the peace in the town of Nueva Concepcion, about seven miles from the bridge, that the area was not under his jurisdiction. In the neighboring jurisdiction, seated in the village of La Reina, about 11 miles from the bridge, the justice of the peace said that he had been informed of the killings and the burials by his "commissioner" that morning.

He said that he had neither examined the bodies nor authorized the burial but that no investigation of the incident was being ordered because of "clearly insufficient evidence."