Awaiting what is thought to be an imminent British attack on the Falkland Islands, a recently reunited Argentina appears to be reverting to paralyzing partisan rivalries and internal mistrust that so often have characterized its past.

President Leopoldo Galtieri, the trim, white-haired general who embraced the style of a populist politician in the early days of the crisis, has disappeared from public view. The Navy and Air Force commanders have retreated into a maze of private meetings in secluded headquarters, reported only cryptically to the public.

Argentina has been ruled since Sunday by a faceless, voiceless entity, the military junta. Each night, the junta is reported to be meeting in the heavily guarded presidential palace. And early each morning, the junta has issued several short, terse paragraphs that have increasingly become the government's only formal contact with the country: "The military junta communicates to the Argentine people . . . ."

"The moment of natural euphoria is passed," a government statement said yesterday. No longer is Argentina a country rejoicing in an assertion of power and nationalism, or gathering in flag-waving unity to cheer on its military government.

Instead, Buenos Aires is permeated by a sense of oncoming disaster, and the government itself, suddenly hidden behind the public facade of the junta, seems to be besieged by external and internal events over which it has lost control.

"This is beginning to look like it could be come a nightmare for our country," a high-ranking political source said privately. "Argentina could be entering a period of turmoil from which it will take many, many years to recover."

The virtual silence in Buenos Aires as the crisis reaches a climax has become penetrating. No information is offered outside of the communiques delivered each night while the city is asleep. Galtieri and the other junta members no longer tour military bases or chat informally with the press.

Last night, the junta suspended all radio and television transmissions at midnight. Screens remained blank for several minutes while viewers were urged not to leave their sets. Then, with no explanation, normal programming resumed. It was said later that the junta was conducting an "exercise."

Within the government, silence, fear and bitterness have become palpable. Political parties are attacking the government openly. Ranks of moderate brigadier generals in the Army are quietly preparing for an overthrow of the junta in the event of a military defeat, according to informed Argentine sources.

The country's labor unions called a mass rally Monday against the British threat. The crowd was relatively small, and the chants were aggressive and obscene. For the first time since Argentina invaded the Falklands April 2, the hatred was directed at the military government as well as external enemies.

Groups of youths, linking arms and leaping up and down before lines of police, shouted the singsong rhyme that had become the trademark of increasingly threatening antigovernment demonstrations before the invasion: "It's going to end, it's going to end, the military dictatorship."

After promoting for weeks a new spirit of national unity that was hinted to be the basis of reopening the political order, the military government has returned to a policy of bluntly enforcing internal order. The atmosphere of suspicion and fear that characterized the first years of the six-year-old military rule appears to be slowly reemerging.

Police are stationed at dozens of street corners here at night, and armored wagons filled with blue-uniformed troops and automatic assault rifles are stationed at strategic points. Movies considered inappropriate--"Coming Home," "Z," "Chariots of Fire," are quietly disappearing from theaters.

Police agents, dressed in civilian clothes, have been circulating through residential neighborhoods, noting the precise locations of British citizens and foreign journalists. Newspapers, after a period of freedom, are once again exercising self-censorship.

Among common citizens, there is a clear current of panic. Banks have continued to lose deposits massively as consumers empty their savings accounts and safety deposit boxes. A leaked government report said yesterday that food purchases in the capital had increased 20 percent since the confrontation began, and a plea was issued for an end to hoarding.

Each day this week, the cobblestoned pedestrian avenue through the center of the commercial and business district has been jammed with knots of people debating the military situation and the country's future. Last night, a large crowd still blocked the street outside an old newspaper office, where news bulletins are posted, long after the display windows were covered with iron shutters.

Frightening rumors have begun to take the place of the government announcements that now are curtailed. It is widely said that there were a number of unreported casualties in the Argentine attack on the Falklands and South Georgia, a report that is backed by news from Britain.

There is no government response to these reports, other than references to the "psychological war" that the junta declared last night is being waged between Argentina and Britain. Perhaps, it is suggested in some circles, the dead from the military's war with Britain will be like the estimated 15,000 who "disappeared" during its war against internal opponents: they simply will not be accounted for.

There was to be a communique issued by the junta here last night, and the press spent the night waiting in the presidential palace. But the statement never came.

This morning, the daily portion of official news, in three paragraphs, was released: "The military junta communicates to the nation that, in accordance with the attitude of the English government, it forsees the execution of military operations in the area of the Malvinas Falklands in the upcoming 24 to 48 hours."

And on that note, the silence in Argentina resumed.