Argentina's military government, widely accused of secretly imprisoning or murdering thousands of its citizens, has issued a decree that would allow all such missing persons to be declared legally dead without official explanation.

The action, which was condemned by several European governments and the Vatican, could effectively close the one legal avenue available to relatives of those who have disappeared.

As many as 6,000 families have filed court petitions demanding that the government be forced to release any information it may have on the missing persons. While such cases have been rejected repeatedly by Argentine courts on technicalities, the fact that they are rejected without resolution has put the government in the uncomfortable position of appearing to cover up.

Local and international human rights agencies such as Amnesty International estimate that anywhere from 8,000 to 20,000 Argentines have "disappeared" since the current rightist military junta took power in March 1976 with a vow to rid the country of lefist terrorism and subversion.

Despite frequent witnesses to alleged abductions by government security officials and sworn testimony of many who have found their way out of Argentine prisons, the government has maintained that it has no information on the missing people.

Emilio Mignone, a leader of Argentina's Permanent Assembly for Human Rights, in a telephone interview today, called the decree "a final solution, like Hitler's," to the government's continuing problem of trying to explain the disappearances.

The Swedish government said after the decree was proposed three weeks ago that "any plan to declare thousands of persons dead through this law would be considered appalling."

The new law was put into effect Wednesday, just four days after the Inter-American Human Rights Commission arrived in Buenos Aires for a two-week inspection of the human rights situation in Argentina, which is widely considered to be the major violator of human rights in Latin America currently.

Under the decree, a judge can declare a missing person dead 90 days after submission of a request either by the family or the government -- providing no evidence is produced indicating the person is alive.

The previous law entailed a five-year process of declaring a person dead. During that time the family had no pension rights and was subject to other legal difficulties. The government has said the decree was decided upon with these difficulties in mind.

Since the arrival of the rights commission, hundreds of relatives of missing persons have lined up each day to present information.Leaders of the Peronist Judicialist Party read a strong statement last night accusing the military government of systematically abusing human rights.

The Peronists said the military has used "the fight against terrorism as an excuse to impose terrorism by the state."

When the military, headed by current Poresident Jorge Videla, overthrew the last elected government, headed by President Isabel Peron, left-wing groups, were in the midst of a guerrilla war that the Peronists were either unwilling or unable to stop.

The military government has always denied responsibility for the persons who have disappeared since the coup, maintaining that they either secretely left Argentina, went underground or changed their identities.

But there are witnesses who say they can identify police or military intelligence officers who seized some of those who have disappeared. Some who were missing have reappeared, saying that they were kidnaped, jailed, tortured and then released from military camps or prisons.

While always denying any knowledge of such cases officials have said publicaly that there may have been "excesses" during the military's "dirty war" against terrorism.

There also have been reports, denied by the Argentine government but given credence by a recent State Department statement, that the military government maintains secret camps where at least some of those listed as missing are being held.

Diplomatic sources and human rights activists have suggested that the government may not want to acknowledge that some missing persons are alive because the military does not want guerrillas at large to know who is providing information to the authorities.

It is widely thought, however, that most of the missing persons are dead. Bodies -- their heads or hands amputated to prevent identification -- wash up from time to time along Argentina's coast.

The work of the human rights commission has been hampered by police raids several weeks ago on local rights groups that had gathered evidence on the disappeared persons. The police confiscated masses of information, acting on a warrant from a judge who was hearing one of the few rights cases to reach a court.

Under the new law, the court asked to make a death finding is supposed to run newspaper ads for five days asking for information about the disappeared person.

Rights activist Mignone said, on the question of pension rights, that none of the relatives "is asking for pensions. We are asking for information about those who have disappeared." Saying the new law is "without precedent in any country," Mignone said a group of relatives will file suit next week asking Argentina's supreme court to declare the new law unconstitutional.