The young lingerie salesman in Tehran's grand bazaar turned away from his customers and began speaking in loud, agitated Persian.

"Ask him," he twice demanded of my interpreter, "why Carter doesn't give us the shah."

Before the translation was completed, he faced me, his pained mien now softened by a smile, and said in perfect English, "Please, sir, will you have a cup of tea?"

An American in Tehran gets used to mixed signals. In a day's travel, he may be assailed with radical slogans and complimented for his taste in clothes, physically threatened and invited to dinner.

Examples of this dichotomy abound in the Iranian capital, transformed in less than a year from a Middle Eastern welcome mat for Americans to the world center of American hatred.

Consider this juxtaposition: On Talaghani street, the U.S. embassy is ringed by chador-draped women and khaki-clad men, all waving pocket-sized Korans and chanting anti-American slogans.

A few blocks away, street vendors do a brisk business in Elvis Presley tapes and Charlie Chaplin posters, and lunchtime crowds of Iranians pack into a Kentucky-fried chicken shop.

On Christmas Eve, a teen-age demonstrator at the U.S. Embassy walks over, his jacket pockets portruding, and points in my direction, gangster style.

"Bang, bang, CIA," he mutters in faltering English.

Then laughter and a few more words: "Happy Christmas, Happy New Year, death to Carter, death to shah."

OUTSIDE THE EMBASSY grounds that serve as the focus of anti-Americanism, life goes on pretty much as normal. But it is impossible to escape the tensions in a city where Americans are being held captive, where daily diplomacy keeps the nation on the brink of war.

In Tehran's bazaar, a 19th century network of stalls and alcoves, business as always is the order of the day. At the entrance there is a long banner declaring that the union of shoe merchants supports the embassy takeover.

Nightly television broadcasts often include a public-service program explaining how to load and fire automatic weapons in the event of an American military invasion.

Almost every conversation with the shop people or cab drivers turns to the hostage issue. Will President Carter send in troops? Will an economic blockade starve the nation and close down hospitals? How will it all end?

Americans are usually reminded that Iran has nothing against "the people" of the United States, just the government. If the people prevailed, they would surely return the shah, goes the conventional thinking.

The owner of a photography shop, happy to meet one of the scores of American journalists who have turned his city into the world's biggest newsbeat, said, "Washington Post, New York Times, ABC, CBS, NBC, you are all good. Carter makes you lie."

THE FARTHER NORTH you drive in Tehran, the more sympathy you get as an American. There, in the beautiful villas lying at the foot of the Eldurz Mountains, the remaining industrialists and professionals -- many of them American-educated -- talk of the new Islamic government as one might refer to a slightly tipsy maiden aunt.

What has happened to Tehran, they ask, with its Parisian boutiques, seductive discotheques, bars and movie houses?

"I have to hide my Scotch today," complained a millionaire businessman, upset by the ban on alcoholic beverages. "We're back to medieval days."

In a fashionable women's clothing store, the owner was asked if the new emphasis on Islamic purity hurt sales of outfits that show off the female figure.

A customer trying on a sheer blouse overheard the question. "I'll take this," she said in Persian. "Tonight I'm going to show off."

Tehran is a town unsure of its past and unsure of its future. America has a foot in both.

Coca-Cola, the most popular soft drink here, carries labels on both sides of the bottle: one in English and one in Persian.

Many women wear full chadors today but the body-length black veils often drape over tight sweaters, designer jeans and knee-length leather boots.

Iran's armed forces recently held a convention to discuss ways of achieving self-sufficiency from American suppliers of equipment. Lined up on rows of tables were thousands of spare parts and electric components that have been provided over the years by the United States.

A Navy officer, asked whether his country really expected to be able to manufacture the sensitive equipment, brought me over to the clothing section and pointed out that Iranian industry already produced excellent combat boots and socks.

How about the airplane engine parts on the next table?

Return to a pair of woolen socks produced by a domestic firm, "Well, we have to start from the bottom up."