Six former members of the Salvadoran National Guard were turned over to civilian authorities here today as suspects in the 1980 slayings of four American churchwomen.

The heavily guarded suspects were brought by a pair of American-made "Huey" helicopters from the National Guard garrison in San Salvador to this small provincial capital, near the scene of the deaths. A special team of prosecutors arrived in a convoy of armor-plated station wagons with military bodyguards carrying submachine guns.

This long awaited move in the case is roughly analogous to the arraignment process in U.S. courts. Under Salvadoran law, the civil court judge now has 72 hours to evaluate the evidence to decide if it is sufficient to pursue the case further against any or all of the suspects.

Although the judge could ask for still further investigation in the now 14-month-old case before determining whether a trial should be held, informed officials--who have long insisted that public action would be taken as soon as there was enough evidence--now say such evidence exists.

The question of who sexually abused and killed three American nuns and an American lay worker the night of Dec. 2, 1980, has become one of the most volatile of many issues surrounding U.S. support for the military-civilian junta fighting a leftist insurgency here. Relatives of the victims and critics of U.S. policy have charged the Salvadoran authorities with foot-dragging.

The case initially caused the United States to cut off all economic and military aid to the ruling junta, which has been frequently accused of extensive human rights abuses. The Carter administration resumed aid early last year in the wake of a guerrilla offensive but only last month the Reagan administration had to certify to Congress that there was progress in the investigation.

Judge Bernardo Rauda Murcia, who was assigned to the case six days ago after the unexplained resignation of his predecessor, appeared today to be a virtual bystander in his rundown offices as the prosecutors from the capital dictated every detail of the presentation of evidence in the deaths of nuns Dorothy Kazel, Ita Ford and Maura Clarke and lay worker Jean Donovan.

Two of the suspects were led in separate from the others. Among the other four was the sergeant who commanded this former squad of national guardsmen the night of the murders.

Most of the suspects tried to hide their faces from the crowd of cameramen weaving among the uniformed national guardsmen who provided most of the security here. One suspect, wearing a silver crucifix around his neck, cried softly. Another, with long curly hair and a Mickey Mouse T-shirt, glared at reporters.

Former sub-sergeant Luis Antonio Colindres Aleman appeared unconcerned by the proceedings as he read a newspaper and joked with the helmeted guardsmen watching over him. One of the two separately held prisoners clasped his manacled hands as if in prayer.

The two separate suspects were arrested only this year, according to officials close to the investigation. In a formal statement today to the court, the commander of the National Guard one of them had been "captured' at 6 p.m. on Feb. 8, but officials privately conceded this was a "technical" statement. The pair was reported in San Salvador to have been arrested last month.

The other four, including the sub-sergeant, have been under administrative detention by the National Guard since April. According to the letter to the court, they were dismissed from the National Guard yesterday morning, a prerequisite to turning them over to the civilian court, and immediately put under arrest.

Those presented to the court today included Colindres Aleman, Francisco Orlando Contreras, Jose R. Moreno Canjura and Daniel Canales Ramirez, all of whose names have been reported repeatedly over the last few months. The new suspects presented today are Carlos J. Contraras Palacios and Salvador Rivera Franco, both of whom were said by investigators to have made extraofficial confessions and were expected to repeat them formally in front of the judge.

Contreras Palacios, according to one official, is a newly devout Christian alleged to have confessed to having participated in the actual murder of the churchwomen.

Rivera Franco, this official said, was a member of the National Guard patrol that stopped the nuns' Toyota van near the El Salvador international airport that night.

Two other guardsman who were detained last April, reportedly because of confusion about some of the names of the alleged killers, were not brought before the judge today and according to one investigator will probably be phased back onto active duty.

The statement by the National Guard commander, Gen. Eugenio Vides Casanova, also mentioned a "witness" named Julio Cesar Valle, a former guardsmen, who "could not be found." Valle was identified by officials as a guardsmen who is alleged to have picked up the killers and driven them back to the airport after they burned the nuns' van.

Reporters who were able to glimpse the judge's offices saw officials formally registering and reporting three bits of material evidence that were also mentioned in Vides Casanova's statement: a tire and a jack believed to be from the Toyota van, and a burgundy-colored skirt with a label reading "Exclusivas M. Gabriela" from Guatemala.

Officials familiar with the investigation said the skirt was sold to a cleaning woman at the suspects' barracks and may have belonged to one of the victims.

It is not clear why a shell casing from a military 7.62 mm bullet--which was found at the scene and which U.S. ballistics tests established as having been fired in a rifle issued to Moreno Canjura--was not presented here today.

The Salvadoran legal system is a maze of concepts alien to most U.S. citizens. The suspects could not be brought here today, for instance, until they had all been formally dismissed from the National Guard. A soldier cannot be tried in a civilian court, but a military court cannot try anyone for murder.

If the judge feels satisfied that there is enough evidence to convict, he becomes, in a sense, almost a prosecutor and may then move ahead to the trial stages of the case, over which he would continue to preside.