On any given day, people from all over the world arrive in Washington.

Some come as visitors, to work or to play, to protest or plead. Others plan to spend a few years here as students, diplomats or political exiles. And then there are those who come to stay -- the growing numbers of immigrants, refugees and the aliens who settle here illegally.

Some fly into Dulles on the Concorde; others are smuggled across the 14th Street Bridge hidden in the backs of pickup trucks. They arrive on buses and trains, speaking Spanish and Vietnamese, French and Hindi, or the many languages of Africa.

They are making Washington something it has never been before -- an international city.

The signs of change are everywhere. Washington is not yet Paris or London, not by a long shot. But it is a far different city than the Washington of 20 or 10 or even five years ago.

"It was a prairie, just a prairie. There was nothing here, really," said one businessman who has watched closely the transformation of the nation's capital into a world capital. He talked about his impressions of Washington in the 1950s -- a cultural backwater, virtually nothing in the way of theater, certainly not a center of the arts. A city, in his eyes, of the most parochial American tastes.

"But now," he said, "just look around you. It doesn't take a genius to see that Washington is an international city."

World famous opera-companies -- La Scala of Milan and the Vienna State Opera -- perform for packed houses at the Kenedy Center, and return home without playing New York. The soft murmur of foreign accents can be heard as visitors stroll through the new Hirhhorn Museum, the new East Building of the National Gallery of Art and the new National Portrait Gallery.

Parisian-style sidewalk cafes in many Washington neighborhoods now outnumber luncheonettes, and scores of new restaurants have opened in recent years offering everything from Ethiopian to Thai cuisine.

The faces one sees on the streets, the languages one hears on the subway, even the names encountered in the telephone directory, are daily reminders of the myriad countries from which Washington now draws its population. The world seems to be moving into Washington's backyard.

The process began in the 1950s as dozens of emerging nations, eager to ratify their membership in the international community, opened embassies here.

Even as Washington received hundreds of these new international envoys -- swelling the number of diplomatic families here to its present 2,300 the U.S. government sent thousands of Americans to foreign lands. They went off by the planeload, representing not just the State Department but also the Agency for International Developement, the International Communication Agency and a swarm of other agencies.

Many of these Americans came back to Washington in the 1960s and 1970s not just with the usual collection of artifacts, but with new international intersts, tastes and friends.

The internationalization of Washington of such organizations as the World Bank, the International Monetary Fund, and the Pan American Health Organization, whose headquarters staffs here have expanded sharply.

International organizations here today boast more than 4,000 foreign employes, contributng a melange of sarongs, dashikis, turbans and flowing Arab gowns to the rush-hour street scenes along K Street and Pennsylvania Avenue.

The pace of change increased not dramatically in the 1970s, when a sagging dollar suddenly made Washington an increasingly attractive vacation option for European and Japanese visitors.

By the thousands, they came. And Washington today makes them welcome.

Foreign tourists casually handing a stack of Swiss francs or German marks to the cashier of a Washington hotel as they check out, however, get a far warmer reception than they would have if they attempted to pay their bill in a foreign currency in the 1950s.

As they came to visit, many wealthier tourists have decided Washington was a place where at least some of their money might like to find a home. The price of a house in McLean or a Watergate co-op may strike many Americans as high, but real estate here looks like a bargain compared to prices in Paris or Munich.

At the upper income levels, foreign investors have begun to shape and stimulate the patterns of investment, the tastes and the fashions of the city.

This is only one side of the internationalization of Washington.

There were other forces at work in the 1970s as well -- the flood of Southeast Asian refugees, increasing concern over the problems posed by illegal aliens, the rising voice of Hispanics in local politics.

These currents, also part of the internationalization of the city, demand public attention, for they touch the everyday life of every person who considers Washington home.

At the lower levels of the pay and prestige scale, Washingtonians now compete for jobs with foreign-born workers who bring a new sense of possibility and perseverance to occupatons that Americans too often scorn as too menial. The mom and pop who run the corner grocery store in Washington today are as likely to have been born in Korea as in the District of Columbia.

Washington's schools now must cope with even greater numbers of children for whom English is a second language. Five percent of the total public school enrollment in the Washington area -- 21,628 pupils -- are citizens of foreign countries.

Washington's streets, with increasing frequency, are the scenes of protest over issues of great importance, mainly to countries that most Americans would have trouble locating on a map.

The city's racial and ethnic frictions no longer are plainly etched in black and white, but rise from the complex tones of clashing cultures and new, often alien values.

Some essentially foreign-born groups are seeking successfully to influence the tenor and policies of the local government -- which is now their government as much as anyone else's.

The precise number of new immigrants here is hard to pinpoint. Estimates of the foreign-born population now living in the metropolitan area who are not American citizens range upwards from 100,000, but it is a population that is so mobile, so new and in some cases so hidden from the government that accurate figures do not yet exist.

In the District of Columbia, for instance, nearly 20,000 foreign residents, visitors, students and refugees from 122 different nations filed annual registration cards with the U.S. Immigration and Natualization Service in 1979. That was up about 5,000 from the year before. In all of Maryland, 57,000 registered, with 55,000 in the state of Virginia.

Uncounted thousands more -- naturalized U.S. citizens, illegal aliens, people who simply forgot to send in the INS registration cards -- do not show up in statistics.

The demographic breakdowns that do exist show the international population of Washington as an amazingly variegated mixture. No single nationality or class predominate among the new immigrants here, though some are more visible then others.

There are few enclaves of distinctive national groups, but with the possible exceptions of the Hispanics in Adams-Morgan and Mount Pleasant, Washington is without "ethnic" neighborhoods like those of a Boston or New York or Baltimore -- and Washington is not becoming an "ethnic" city. Its Chinatown remains minuscule. Its Little Italy is nonexistent.

Even in Adams-Morgan, at least one reason why Hispanic leaders have been unsuccessful in attempts to have the neighborhoods deemed the "Latin Quarter" by city officials is because it is really more of a melting pot, with Jamaicans as well as Bolivians, people from Ethiopia as well as those from El Salvador and a thorough blend of longtime Africans, too.

There are apartment buildings in Arlington, Prince George's and Montgomery where the population is equally diverse -- a jumble of nationalities trying to sort out their lives as they start them over in a new world.

In a Takoma Park apartment the teen-aged son of a recently arrived family of Vietnamese refugees learns English from a Portuguese boy whose parents fled Angola after the revolution there.

Washington has reached the stage where private groups and the city government have begun not only to recognize its international character but to try and build on it. Mayor Barry for instance, has a full-time staff member whose business it is to promote the development of international investment in Washington.

As a Londoner who now makes his home here remarked recently over a cup of coffee (in an Italian restaurant served by a Central American waiter), "Great cities are not made. Great cities become."

And what Washington becomes, ultimately, will be determined not by grand plans but by the unguided, collective impact of the individuals and families who arrive here -- their successes, their failures, their expectations.