These are the last of the golden days in Washington, that time when late fall flowers still bloom in the parks and the stores begin to fill with shoppers. The holidays approach, the pace slows perceptibly as the politicians either scatter or plan their trips, and Washington takes on more of the air of a prosperous smaller city than a world center.

It's much more than this, of course, as anyone could see yesterday. In only 24 hours the look and feel and sound of Washington had been transformed.

On Connecticut Avenue, the day before, I had been struck by the extraordinary display of affluence in the way the crowds were dressed, in the profusion and range of goods in the windows, in the luncheon lines gathering at the expensive restaurants. There had been only one small reminder linking Washington with the world: the soundlessly flashing electronic sign on a building facing the small triangle where Connecticut, 18th and N intersect. Second by second the sign was ticking away the numbers of the world's population. "Every minute another 172 persons," it proclaimed, as the light green numbers kept changing, changing, changing, always increasing. 4341103434 . . . 5 . . . 6 . . . 7.

Now, a day later, helicopters droned overhead, circling back and forth over that some section. They have an ominous sound when they patrol that way. Cirens are more familiar; they do not evoke the same sense of menance.

In Lafayette Park, where the yellow mums are at their peak, the lunchtime crowds of the previous day had presented a picture of peace and harmony, complementing the quite splendor of the White House across the street. Now there was something ugly in the air. Frightening would be a better word.

An urgent, insistent voice was booming across the park through a loudspeaker. The voice was calling the cues, and the demonstrators were responding in mass: "Shah is a butcher, down with the shah." In the center of the park, a few feet from Andy Jackson's statue, the one that has Old Hickory heroically waving his tricornered cap from a rearing horse, a drum had been set up. Boom. Boom. Boom. It beat out the rhythm for the crowd: "Shah is a butcher." Boom. Boom. Boom. "Down with the shah." Boom. Boom. Boom.

At Lafayette's statue, the bronze long since turned green with age, three young men had climbed to the top and unfurled a red banner. They struck noble poses for the camera crews circling through the long lines of marching, chanting, shouting protesters. Mounted police were posted around them. On the White House lawn, Secret Service guards had put on gas masks.

But none of this gave that scene its special, fearsome flavor. That came solely from the way the protesters were dressed. They all wore masks. There's something primal about the sight of people wearing hoods these days. It stirs memories of terrorism, and it vividly brings home reminders of how international issues intrude on even the most tranquil of times in the capital.

And of something else: The perils and problems of official travel in this era of increasing violence and political cynicism, and the sophisticated ways in which competing sides seek to use the mass media for their own interests. The shah's visit to Washington is only the latest example. He arrives, and immediately gives a round of "exclusive" interviews with the TV networks including the smaller Public Broadcasting System.

As his word goes out, the clamor of his opponents to attract attention increases. Yesterday before the violence, I was walking through the crowds of demonstrators by someone seeing my press credentials and notebook. Didn't I want to interview the students? Didn't I want to speak to this or that leader? Didn't I want the literature they were supplying to passerby? What was I there for then? The message of the demonstration had been set out before me and I, apparently was not delivering it.

The shah and violence notwithstanding, delivering the mass massage is the aim of all demonstrations these days. And that intention isn't limited to foreign visitors, either. We're about, it seems, to witness another kind of demonstration before too many weeks are out - an international one, played to a home audience. The word is that the President still hopes to make that continent-continent trip, one that brings with it built-in intensive media coverage and world attention.

It comes as no surprise to find a certain defensiveness in the White House about the President's planned - and then postponed - visit to nine countries in 11 days. It was, you'll recall, supposed to begin next week, and then was put off. The stated purpose was to give the President time to closer personal attention to the fate of his congressional energy package.

Both the timing of the original announcement and the postponment brought strong criticism of the White House's handling of the trip. Announcing it immediately after Bert Lance's departure as budget director was seen by some as a device to improve the political atmosphere. Cynics, of whom there are a few in Washington were shortly referring to it as another presidential case of "when in trouble, travel" psychology.

Planning went forward, and the White House dispatched its advance teams around the world. They were accompanied by media representatives who wrestled with establishing TV facilities in some of the most difficult places a President has been. For example, there have been no previous successful commercial TV broadcasts from the African nation of Nigeria. Neither had color TV ever been broadcast out of India. These intricate, exhausting and expensive logistical arrangements, to say nothing of the diplologistical arrangements, to say nothing of the diplomatic ones, were nearing completion when the word came that the trip was off, temporarily or otherwise.

That produced another round of criticism. "The President's attempted too many things," one eminent foreign affairs expert in Congress said. "And now he's done this very odd thing on this trip. If he'd put the proper time earlier on his energy bill in Congress, he could have gotten that through and spared him this embarrassment."

The Foreign reaction was muted officially in public, but often critical in private. Criticism from Brasilia, one of the capitals he planned to visit, was fairly typical of what Washington Post correspondents heard in many of the nine countries. The Brazilians were perplexed, our correspondent reported. They didn't doubt that the energy package was the reason for the postponement. But they couldn't understand how he could have scheduled a trip knowing his energy program was in trouble in the first place. "It's just one more sign of his political immaturity," one Brazilian diplomat said.

Naturally, the White House views this differently: The planned visits are serious and substantive. They offer the President a chance not only to see some of the old European capitals, but to focus on the problems of nations and continents in flux. The educational value, and the personal experience derived, are incomparable. "You can read about it," says one presidential aide, "but to see it is unforgettable."

No doubt that's true; all recent Presidents surely would agree. But there's another truth to these flying national caravans. Behind the ceremonial pomp, the flags and parades and booming salutes, the formal banquets and toasts and cheering communiques, a larger purpose exists. It's simple enough. Merely to see some leader or some country isn't enough. You have to be seen and heard by the nation and, if you're a president, by the world.

The shah understands this. He's been getting his message across on 11 previous state visits here. He knows personally how best to handle the diplomatic demonstrations of statecraft. It's just that now the other kinds of demonstrations are getting more difficult, and the official message becomes more murky. That could be true not only for shahs, but for presidents.