Large posters with bold black letters were plastered around the grandiose Plaza De Mayo, the city's central square, and the message, although translated into Spanish, had a familiar ring:

"He has plundered our seas, ravaged our coasts . . . He is at this time transporting large armies of foreign mercenaries to complete the work of death, desolation and tyranny already begun with circumstances of cruelty and perfidy, scarcely paralleled in the most barbarous ages. . ."

Originated by a local military reserve association, the poster asked, "Reagan and Haig: Have you read the preamble to your own Constitution?"

The words, taken in fact from the U.S. Declaration of Independence, summed up the emotions that burst forth at churches, town squares and street corners today as Argentina commemorated the 172nd anniversary of what it calls "the cry of liberty."

It was on May 25, 1810 that the city fathers of Buenos Aires, emboldened by victories against the British a few years earlier, gathered in the Plaza De Mayo and called on the city council to depose the Spanish viceroy.

On this quiet holiday, as the shadow of war hung over the nation, Argentina seemed to savor the memory of past triumphs. The three members of the junta raised flags and sang the national anthem in separate ceremonies around the city.

Observing a moment of silence for Argentines who have died in the conflict over the Falklands, President Leopoldo Galtieri said that "on our Malvinas Islands, the sons of our land, the Air Force, the Navy and our Army, in the trenches, on the attack and in their quarters must be singing the national anthem together with us."

Crowds thronged the Cabildo, the colony's first seat of government, now a museum, to ogle the pistols and bayonets used in the British invasions of 1806 and 1807. The morning paper noted that the "Plaza Britania," one of the city's main squares, was renamed the "Argentine Air Force Plaza" yesterday to recognize "the Air Force's brilliant fight. . .in the defense of our sovereignty."

At a solemn Te Deum in the cathedral, attended by the junta, top government officials and the diplomatic corps, The Rev. Raul Rossi scolded the "small although powerful part of our world which was scandalized and sanctioned us for having dared to recuperate the price of our own blood, what was and always will be ours . . ."

More than 2,000 flag-waving men, women and children crowded the sidewalks for a glimpse of the junta as each of the three commanders climbed out of his separate limousine. In the cathedral, dressed in the full regalia of the Army, Navy and Air Force, the trio walked down a red-carpeted aisle under an archway of swords raised by soldiers in Napoleonic era uniforms.

A blind choir and a chorus of schoolboys sang "Gloria in Excelsis" as the three passed through the rows of male officials in the left pews, wives in the right pew to take their seats on three red silk-covered chairs in front of the altar.

A few blocks away, in a dark corner of Santo Domingo Church, a group of women said the rosary before a gold-crowned statue of the Virgin Mary. "Pray for us, our Mother of the Rosary of the Defense and Reconquest of Buenos Aires, and patroness of the Malvinas Islands," the women repeated in unison.

It was before this very statue of the Virgin that the gallant Captain Santiago de Lenieres prayed during the British attacks, and it was to her that he donated the standards of the British troops he defeated in 1806, according to a marble plaque near the altar.

"This one belonged to the invincible Scottish regiment," said Elina Del Punta, a retired school teacher, gesturing contempuously toward one of the frayed and fading banners on the wall. "They were supposed to be invincible, but we've got their flag. We kicked the British out before and we'll kick them out again."

Wherever one went today, Argentines struck up conversation--anxious, it seemed, to talk of wars past and present. In the Cabildo courtyard, a dozen men and women, gathered in a knot, all talking at once.

"What Haig did to us was treachery," said del Punta. "The word Haig is a dirty word. What a nerve! They think we're Indians from Africa. But the British are decadent. It will be a long fight, with many lives on both sides."

"The people are united," said Elina Yacby, Del Punta's daughter. "We have fought against the British since 1806."

Rosa Richert, a housewife, interjects, "Reagan is supporting them because he wants military bases in the south." A schoolteacher, Yolanda Caivano, said her students are writing the soldiers, "and if you could read the letters, you would cry."

The parade and the dancing in the Plaza de Mayo, which usually occurs on this holiday, were canceled because of the war. Although the holiday celebrates this nation's first move toward independence, what is called here "the program of institutionalization," or a gradual move toward democracy, seems also to be a casualty.

In an editorial, La Nacion, the establishment paper, noted that the colonists exercised authority under King Ferdinand VII and declared that "the unavoidable return to the path of the Constitution . . . should be postponed because for the moment the absolute priority is achieving peace with the recognition of Argentine sovereignty."

Across the square, Selva Fraguglia, 22, pregnant, clutching a baby and a two-year-old in her arms, seemed uncertain. She sat on the steps of the cathedral, her clothes filthy, and sewed tiny little blue and white ribbons, the national colors, as lapel pins. "The Malvinas are Argentina's," she said, "but I'm scared for my children."