The Renaissance Center, whose gleaming towers of concrete and glass on the bank of the Detroit River once symbolized the rebirth of Detroit, is bankrupt, its hotel little more than half filled, many of its designer shops now empty and one of its newest office towers vacant.

One of the largest downtown development projects in history, the Renaissance Center has been losing money since the day it opened in 1977. Its owners, including the locally based Ford Motor Co., have been trying to sell the entire place for months. The latest blow came last week, when Ford and the other owners failed to make their monthly mortgage payment, sending the $420 million complex into default.

"We've sort of come to the end of the line," said Richard Routh, spokesman for the Ford subsidiary that owns 65 percent of the Renaissance Center. "We're not going to close the place, but we've exhausted our credit, even with our own company."

All this is a far cry from the summer of 1980, when the new center proudly played host to the Republican National Convention. Then it seemed that Henry Ford II's idea for rebuilding riot-scarred downtown Detroit was near reality. Mayor Coleman Young was hailed as an urban leader of great vision. Newspapers and magazines were filled with stories about Detroit rising from the ruins.

The long recession and the depressed auto industry have sent Detroit into a tailspin so serious that the governor of Michigan has declared the city to be in "a state of human emergency."

Unemployment has soared to 19 percent of Detroit's work force, and more than 400,000 people, a third of the city's population, are receiving some kind of public assistance. Two of every three jobless workers here have exhausted their benefits. Soup kitchens are trying to feed 20,000 people a week.

Even in better times, the riverfront Renaissance Center was plagued by poor marketing, a confusing circular layout, a saturated office space market in Detroit and a surplus of overpriced boutiques ill-suited to the city's blue-collar population. Some critics contend that the money could have been better spent on jobs or inner-city housing.

"Ren Cen really doesn't appeal to the average person in Detroit," said Denise Jacobs, a community activist who was laid off from her job at a metal stamping factory. "It's too expensive. Everything is just sky high. It's an eyesore that was basically built for the mayor.

"It's not going to alleviate the main problem here, which is skilled laborers who can't get a job," added Jacobs, who now manages an auto paint store. "We can have all the maid and busboy jobs in the world, but these are minimum-wage jobs. Detroiters are used to making decent money. They don't want to be janitors and sales clerks."

Still, city officials insist there is a substantial silver lining. More than 11,000 people work in the Renaissance Center, which has prompted private development all along the Detroit River. Several prominent businesses have decided against joining the exodus to the suburbs.

Nearby apartment buildings that were going broke now have long waiting lists. A few blocks away in "Greektown," a once-dingy row of abandoned buildings, there is a cluster of boutiques and chic restaurants serving filo and moussaka.

These changes did wonders for the city's image. More than 750,000 convention delegates came to town last year, up from 430,000 in 1977, pouring $175 million a year into the city's economy.

"Words can't describe the excitement this has brought to downtown Detroit," said Robert McCabe, president of Detroit Renaissance, the nonprofit group that helped create the center. "You wouldn't believe the 'before' and 'after' pictures. People no longer think Detroit is going to die. The center has done exactly what we said it would do in stimulating downtown."

McCabe said the center was overwhelmed by record interest rates and unemployment. "You could see the handwriting on the wall," he said. "Detroit is in the worst depression since the Great Depression."

Although the center was built entirely with private money from Ford and 50 other local businesses, the mayor has been sowing federal grants in the ground surrounding it. The money is helping to build a three-mile, elevated "people mover" and two luxury apartment towers with a yacht basin and health club.

"You've got to understand where we started from," said Emmett Moten Jr., Detroit's director of community development. "We did this in the face of 20 percent interest rates, Chrysler going bankrupt and GM and Ford losing money. Now the town is awake and alive. This is the place to be in the summertime, with jazz concerts and ethnic festivals."

Moten maintains that 80 percent of the city's federal development grants are spent in residential neighborhoods. "We have built more first-class subsidized housing in the last five years than any city in North America," he said.

But no one denies that the downtown shopping district is in deep trouble. The last major downtown department store, J.L. Hudson's, will close its doors in two weeks. Hudson's hung onto the century-old store for years as sales plummeted, waiting for a promised downtown shopping mall that never arrived.

The Renaissance Center, hidden by concrete slabs, has lost $140 million. High-priced shops such as F.A.O. Schwarz and Hoffritz Cutlery have not done well, and the retail space remains 40 percent vacant. Halston's, one of the few designer shops left at the center, has cut back on $300 silk dresses to push $40 sweaters and $60 skirts, "things that appeal more to working women," according to saleswoman Kathy Fisk.

Nor have conventions been enough to fill the Westin Hotel's 1,400 rooms, two ballrooms, revolving rooftop restaurant and sunlit atrium lobby.

The latest addition to the center, a pair of 21-story towers partially financed by Rockefeller Center, hasn't fared much better. Ford, which already has 1,700 employes at the center, signed up to rent one of the towers, then backed out as auto sales fell, leaving the building vacant.

Last spring Ford agreed to sell the hotel, shops and four original buildings to a group of investors headed by Chicago accountant Theodore Netzky, who mainly were interested in a big tax writeoff. But they could not raise enough capital.

The financing already had been renegotiated in 1980, when the four life insurance companies holding the mortgage agreed to defer about $10 million in payments until the begining of 1983. Those came due this month, and after a 10-day grace period the owners simply ran out of cash.

"It was no surprise," said an official of one of the lending companies. "They were looking for a sale, the sale didn't come through. They were looking for more business and that didn't come through. When the time came to pay the piper, they couldn't pay."

But another insurance company official cautioned that his firm is in no rush to foreclose. "I don't know that we want to take it over," he said. "Detroit is a one-industry town, and it's in deep trouble."