"I'm heartbroken," Duke Zeibert said as he looked across the crowded tables of his restautant on the corner of Connecticut Avenue and L Street -- the "best corner in the City of Washington," as he calls it.

"I've seen the whole world go by here. The whole world at one time or another has passed through these doors." Zeibert rattled off a list of political, sports and media celebrities. "I've had a romance with this place for 30 years and I'm heartbroken that I have to move," he said, "but it looks like I'm gonna have to."

Zeibert is not alone. Sholl's Colonial Cafeteria, Brooks Brothers clothing store, consumer activist Ralph Nader, journalist Elizabeth Drew, The Washington Monthly, language and business schools, international and philanthropic organizations: every shop, every office and apartment in the vast old La Salle Building will have to be bacated in the next several months.

Though some of the commercial etsablishments, liek Zeibert's and Sholl's, are Washington institutions, and the residential tenants represent one of the most fascinating cultural mixes in the city ("You can't ride the elevator without hearing three different languages," remarked one office worker), all have to make way for a massive new complex to be built by the same peopel who gave us Maryland's White Flint shopping mall.

Just what this complex will be like, and even the question of when the La Salle's demolition will begin, is something of a mystery. Owners Theodore N. Lerner and Albert Abramson were not talking about the project last week, and the La Salle's manager said he was "under strict orders" not to say anything to reporters.

The partnership agreement between Lerner and Abramson calls for a hotel, offices and "other commercial facilities." Style and substance aren't explained. But since Lerner, in addition to White Flint, is responsible for Tysons Corner, Wheaton Plaza and Landover Mall, most informed people are guessing the new building will be a further attempt to make that stretch of Connecticut Avenue -- near the Mayflower Hotel and just four blocks from the White House -- one of the sleekest, chicquest shopping districts in the city.

Most visitors to the corner don't know of the imtpending transformation yet, and those who were tole by a reports didn't sound happy about the change."I like this corner," sighed one young woman who often shops in the area. "Oh gawd, they're going to put a lot of palm trees and fountains in it."

The tenants -- who are enamored of the building's low rent as much anything -- tend to look on the change as a glum inevitability.

"It's really very, very sad," said 80-year-old Evan Sholl, whose cafeteria has been serving wholesome food at unheard-of low prices on the same corner for half a century. "We had 10 years with the Metro tearing up in front of us, now it's the best corner in Washington and we've got to move. But I kind, of look at it like this: We've had 50 wonderful years and God has been good to us to lead us to another location."

Others -- most others -- haven't been so lucky.

Of the 96 residential apartments connected to the switchboard in the lobby, a receptionist said, most are now vacant. Those wo are left are still looking for someplace to go before their Feb. 28 deadline for departure.

The commercial tenants and the 270 or so office tenants listed on the building directory are in the complicated process of trying to find similar facilities nearby. It's seemingly an impossible task for most of them, and one that requires much negotiating.

An executive of the Holiday Metro Spa, for instance, said he is trying to make a deal with the owners of the La Salle to move into one of the other downtown buildings they own. The manager of Brooks Brothers declined to talk about the matter except to say, "We're not folding out tents at all. We'll be here one day and someplace else the next."

The situation is further complicated by the fact that many who have offices in the La Salle are not sure just when they will have to move out. They believe they are supposed to leave before June, but the building's owners appear to be dealing with each tenant individually, and are making nothing public.

"Have you heard anything definite?" Ralph Nader asked whtn he was called about the situation. Nader said he thought he had an informal understanding that he wouldn't have to move his offices until sometime in May.

Though Nader knew the building would be torn down when he moved in a year ago -- he signed a short-term lease at a cost of " $5 or $6 a square foot" while other offices in the area cost $24 a square foot -- he waxed vehement on the trend that demolition represents.

"They're squeezing out the citizens groups," said Nader. These little lobbying organizations that are scattered throughout the building and represent the heart and soul of Washington, as Nader described them, simply can't afford the price of office space in gleaming new downtown buildings.

Some tenants have already started to go. Elizabeth Drew, the New Yorker magazine's Washington reporter, talked of sitting in her seventh-floor office and hearing people moving out. "It's something like being in a Chekhov play and hearing the trees chopped down all around you... But the building was too good to be true. It just couldn't be saved."

Drew, who has had her office there since 1967, grew a little nostalgic as she talked about the eclectic collection of tenants who have been on her hall. There have been a couple of spin off organizations from the Rev. Sun Myung Moon's controversial church, she said, "and a lot of unmarked doors. God knows what goes on in there. It's obviously a very strange building."

She especially enjoys, she said, the sounds emanating from the business school near her office, which appears to have a number of foreign students. "It teaches them to type to the time of 'Tea for Two.' It sounds like an old Victrola record." Every time the rattling symphony starts, she said, she and her assistant go out in the hall to listen.

Many of the people who actually live in the building are elderly, having had their apartments in the La Salle since its grander days several decades ago.

One older woman, who asked not to be named, said she has lived there since 1942. One of the reasons she had move in, she said as she talked to a reporter near the entrance, was because of the lobby. "It was beautiful. But look at it now," she said. Where once there was a lofty ceiling and spacious entrance, now there is plastic overhead and a relatively narrow, sterile corridor leading to the elevators.

"The heat is off more than on," she said, and told of wearing a robe and a sweater to bed.

She said she has found another place to live and expects to move out of the La Salle on Saturday. "But I bet I went out over 70 times looking for an apartment. It's hard to get one room down here.

"Some places they wanted to know if I was retired. I felt like saying it's none of your biz... A friend of mine went to a nursing home last Saturday and she had lived here since it was built. Fifty years."

Another woman, who said she has lived in the La Salle for 20 years, likened her situation to that of the Indochinese refugees. "The way everything goes, there are many people who can't find a place they can afford. They're like apartment boat people and we've got so many of them here."

As it happens, there are also real boat people from Indochina who have been given temporary shelter in some of the vacant apartments by the International Rescue Committee, which has its offices in the La Salle.

Wandering the halls in the evening and knocking on doors, one encounters people from all over the world.

In one apartment there is a businessman from Afghanistan, stranded in the United States after the leftwing coup in his country last year. In another is young womn from Venezuela who works at the National Gallery. In yet another, a student from North Yemen.

While most of the occupied apartments are small efficiencies renting for from $150 to $250, some are as extravagantly spacious as anything in Washington.

Mireille Holmes and Fernando Casillas, who owns a printing firm only about a block away from the Las Salle, live in a $750-a-month apartment that takes up parts of two floors. The living room, full of antiques and oriental rugs, has a 22-foot ceiling, a stairway leading up one wall above the bar to two bedrooms.

"You know we'll never find another ceiling like this," Casillas sighed the other night. "We're crushed that we have to leave...

"You should see the rumors that have gone around this place since it was sold. We've heard there may be penthouses in the new building. Listen, I would try to get one."

Owners Lerner and Abramson acquired the property -- which has an assesed value of $9,339,750 -- through a bequest to their Washington Square Ltd. Partnership in July 1977, according to the D.C. recorder of deeds' office. City records do not indicate why or for what considerations, the property was willed to them.

Several tenants of the La Salle said they believe its demolition will begin in June, but some, like Zeibert and the Holiday Spa, said they don't expect to have to move out for almost a year.

Zeibert said he still holds a longterm lease that Lerner and Abramson have yet to buy out. Zeibert wouldn't say how much he is asking, "but they're getting to me and they're getting to my price," he grinned.

Some knowledgeable tenants believe that Lerner and Abramson would like their new complex to stretch all the way to 18th street, but one small landowner stands in the way of such a scheme. Nuncio DiPerna, whose two little Italian restautants occupy the corner of 18th and L streets, said he has not been directly approached by Lerner and Abramson yet, but he said he has gotten some signals, and he's waiting for more.

The restaurants -- Tony's and Lenzo's -- have been in his family for 50 years. The business was started by DiPerna's father, a Sicilian immigrant, and is now owned by DiPerna and his two sisters. Unlike many of the La Salle's tenants, DiPerna feels he's in a position to bargain.

"I don't want to get out of here crying," he laughed. I want to get out of here singing."