The city is lovely at this time of year. A trace of autumn chill is in the air. Sweatered children play games in the handsome parks, laughing and shouting in the sunlight.

The stores are full of high fashion goods. The avenues are crowded with shoppers. But there is a somber mood in the streets. Unsmiling crowds gather at the newspaper offices on Florida Street, scanning the bulletins and rumors of war. There are lines in the banks. People are claiming their savings against disasters unknown. There has been a hemorrhage of deposits in the last two weeks, more than a billion dollars, it is said. The word "panic" is heard.

Social tensions surface as the British fleet draws near. An Argentine woman of British descent is upbraided by a stranger for speaking English to a friend. Two Americans are berated by a taxi driver for joking about the Falkland Islands.

Vendors refuse to sell the city's only English language newspaper. British journalists are reported to be under surveillance. There are outcroppings of anti-Americanism, symbolized by the popular cynicism toward the mediation efforts of Secretary of State Alexander M. Haig Jr. He is suspect in government circles as a handmaiden of the British.

Fatalism is another element in the emotional stew in this time of crisis. An Argentine with military connections expects the war to begin on Tuesday, with land-based Argentine aircraft striking the first blow against the approaching British fleet.

"But then we will lose," he says.

Military commanders are described as "emotional, nervous and ready to die." The sense is widespread that events are out of control.

These apprehensions are inflamed by the official policies of secretiveness. Government authorities are inaccessible or uncommunicative. The British Embassy is vacant. The corps of specialists at the huge U.S. Embassy are mute, obedient participants in a policy of noninformation.

Even the commercial community--bankers and business people--is strangely shy. "If I talked to you," an American bank officer said, "I would be fired. I'm under orders from New York." His institution is one of the carriers of Argentina's $34 billion external debt.

That is the currency of the day--rumor and speculation about military movements, about negotiating postures, about the state of the economy. It is thin stuff for the 450 journalists and television technicians who have come here from all over the world to render the big story, the big scoop.

They are frustrated and at times desperate. The Falkland Islands are sealed off. Military installations are off limits. There are no marching troops or truck convoys in the streets, no "bang bang" of any description. The situation is likely to get worse. Already there are intimations of news censorship in the offing and the deportation of "unfriendly" journalists.

Out of this frustration schemes have been concocted to break the barriers of silence. A tanker will be rented in Brazil to steam through the blockade around the Falklands. A yacht will be obtained from an automobile dealer who promises to reach the islands within five days.

Each effort has thus far been stillborn, which may be just as well. Winds of 40 to 80 knots and 30-foot seas engulf the Falklands as winter approaches. Civilian aircraft and surface vessels have no automatic immunity in zones of war.

From the beginning, there has been a Gilbert and Sullivan aura about the Falkland Islands conflict. That may evaporate quickly if the guns are unlimbered.

"The situation is a frightening one," the Buenos Aires Herald said in an editorial today, "since an armed conflict begins between Britain and Argentina and when U.S. neutrality is cast to the wind, it will be a case of a vastly outnumbered and outgunned Argentina against the drive of the most powerful nations of the West and the help of a superpower the Soviet Union to act as an equalizer in the fight . . . .is apt to be very welcome. This means that in the end, what started out as a reaffirmation of Argentine sovereignty could turn out to be as significant to the overall result as the political assassination which set fire to the fuse which in turn touched off the vast explosion of World War I."