Argentine President Raul Alfonsin has ordered the arrest of six Army officers and six civilians suspected of plotting to destabilize the country.

Announcement of the order early this morning came just hours after the conclusion of defense summations in a dramatic trial of nine former military junta members accused of responsibility for a secret campaign of state terror against left-wing terrorists and dissidents in the late 1970s and early 1980s.

The human rights trial has infuriated right-wing groups here and is thought to be among the factors provoking a spate of anonymous bomb attacks and threatening phone calls that have become almost daily occurrences. All 12 whose arrest was ordered today are known rightists with links to the former military government.

Five of the officers have been arrested. One officer and one civilian are known to be outside the country and two of the other civilians have been detained. The other three still were being sought this evening.

With congressional elections scheduled Nov. 3, Alfonsin has been under heavy political pressure to demonstrate that his two-year-old civilian government can guarantee internal security.

An executive decree authorizing the arrests said the recent attacks, which have struck military installations and the homes of military personnel as well as schools and businesses, have been intended both to cause general chaos and to "try to split the armed forces and civilians during this historical moment" of the consolidation of democracy in Argentina.

Within several hours of the postmidnight announcement of the arrests, a bomb exploded outside Army headquarters in Buenos Aires, destroying an empty wooden sentry box but causing no casualties. Interior Minister Antonio Troccoli told reporters it was probable that acts of violence would continue because "not all the instigators of these actions have been neutralized." He placed responsibility for many of the attacks on right-wing cells having international connections and financed by kidnapings, trafficking in drugs and other crimes.

The 12 men cited in the decree were ordered held for 60 days pending a federal investigation on charges that they "could have conspired to make an attack on the constitutional order." But the alleged leader of the group, former general Guillermo Suarez Mason, is reported to be living outside the country.

Already wanted by both military and civilian courts for a variety of offenses, Suarez Mason was stripped of his rank and expelled from the military last year after failing to respond to a summons about abuses committed during the military's violent campaign against leftist guerrillas and suspected sympathizers. He was formerly commander of the 1st Army Corps, the unit that had jurisdiction over Buenos Aires when military rule was imposed and the crackdown on urban guerrilla operations began in 1976.

The fugitive general since has been linked to trafficking in drugs and arms in Latin America, according to a government official who cited foreign intelligence reports saying Suarez Mason has operated in recent months from Miami and Sao Paulo, Brazil.

Five military officers were taken into custody last night under the decree. They are Alejandro Arias Duval, a retired colonel who served as chief of the federal security office in Buenos Aires until December 1983, another colonel, two captains and a major. Three of the six officers named in the decree are on active duty.

Also ordered detained, but not immediately taken into custody, were six civilians: Daniel H. Rodriguez, a columnist for the conservative Argentine daily La Prensa; Jorge Antonio Vago, editor of a weekly gossip newspaper called Prensa Confidencial; Rosendo Maria Fraga, a lawyer who headed an Interior Ministry advisory group under one of the juntas; Enrique Gilardi Novaro, a former undersecretary of both defense and interior ministries; Alberto Hernan Camps, a cadet at the Military College whose father is a former police chief in Buenos Aires Province awaiting trial for human rights violations, and Ernesto Raul Rivanera Carles.

Since taking power in December 1983 after seven years of military rule by three successive juntas, the Alfonsin government has been plagued by a persistent undercurrent of political violence, some of it against human rights activists.

When the human rights trial of the nine former military commanders began in April, a new wave of violence was reported. Several months ago, the government for the first time launched a series of raids against right-wing extremists, uncovering caches of arms, documents and electronic equipment.

But the close of the hearings yesterday, with a verdict expected before the end of the year, marks only the start of other worries for Alfonsin, among them, whether to proceed with more trials against other officers accused of atrocities during the campaign against leftist guerrillas and dissidents.

Speaking out this month in court for the first time since the start of their trial, the ex-commanders expressed no guilt or remorse for what many Argentines call the "dirty war." They defended the clandestine kidnapings, tortures and killings, which the Alfonsin government said resulted in 9,000 deaths, as a just and necessary response to left-wing terrorism.

The officers' unyielding stance has worked against the national reconciliation sought by Alfonsin. By appearing unrepentant to the end, Argentina's former leaders have reinforced a split in the country between those who view the commanders' past actions as the savage and illegal slaughter of suspected subversives and those who defend what happened as the necessary course for a government that had been plunged into an irregular civil war.

According to recent surveys, about 90 percent of those polled support the trial.

But people are increasingly divided over whether hundreds of lower ranking officers who actually committed the abuses, and in many cases are still on active duty, should be put on trial as well.

Alfonsin is sticking to his announced policy of bringing to justice all soldiers accused of the secret abductions, torture and murders. There are reportedly more than 1,700 such cases awaiting trial. But the government also has said it will attempt to distinguish between those who simply carried out orders and those who did so with excessive brutality, and will seek less severe sentences for the first group. Some influential conservative circles, in addition to most of the military, have had enough and are urging the president and his Radical Civic Union to put the whole matter behind the nation by granting the remaining military suspects amnesty.

Human rights activists are insisting that the judicial process not end with the nine. Others fear more trials could deepen national rifts and damage already low military morale.

"You can't go on dividing the country between the good and the bad," said Guillermo Pena Casares, a lawyer representing banking interests. "I think they should convict those on trial now, then stop the whole thing."

Civil libertarians have applauded the trial for greatly expanding public awareness of the excesses of authoritarian rule, thereby shoring up support for a democratic state. But other Argentines voice concern that the political pendulum may now swing too far the other way, ignoring left-wing excesses.

"There is a danger now of romanticizing the other side, making them appear like poor, hunted children," said Jose Miguens, a sociologist who specializes in military affairs. "But they were not freedom fighters. They were totalitarian. The problem for us is that both sides were nasty, both extorted and both killed in the worst ways."

In their summation statements this month, the nine former commanders and their attorneys used mostly political instead of legal arguments to attack the government's case. They made these points:

*That the junta's success in defeating the guerrillas justified the means used, particularly given the widespread extent of left-wing violence when the armed forces took power in 1976.

*That the antiterrorist campaign was a war that, like any war, cannot, in the military's view, be judged by peacetime standards. The ex-commanders said the fight had certain peculiarities that made it unlike conventional warfare.

*That the trial is political and that Alfonsin is wrong to try the former commanders under a law that was passed after they surrendered power. The former junta members, all now retired, also resent being judged by a civilian court, a move the government ordered after apparent foot-dragging by the military in preparing courts-martial.

*That the evidence against them is weak, notwithstanding the 709 cases of rights violations presented by the prosecution, and that many of those who testified against the former commanders were one-time guerrillas or sympathizers.

The defendants stayed away from the hearings until the federal appeals panel judging the case insisted they be present for the summations.

The notion that the hearings are a political show fostered by subversive elements to avenge their defeat at the hands of the military is widely held in the armed forces. Feelings of resentment and frustrations there run deep, according to Argentines with close ties to the military.

"They feel abandoned, used by civilian society," said Mariano Grondona, a prominent journalist. "It's like someone whose house is being robbed calling a cop and shutting his ears to any shooting, then turning around to accuse the cop of brutality."