Fifteen years ago the Inner Harbor of this famed old port city was clogged by derelict buildings, rotted piers and sunken boats -- a repulsive symbol of the worst of urban decay.

But tomorrow an important event for Baltimore -- and perhaps, by example, for cities across the nation -- takes place on the waterfront of handsomely rebuilt Inner Harbor. Developer James Rouse's Harborplace, a $20 million commercial center -- two glass-enclosed pavilions with 120 restaurants, cafes, speciality markets and shops -- will open its doors.

Baltimore's leaders have great hopes riding on Harborplace. If it has the catalytic effect they're predicting, tens of thousands of residents, tourists, sailors and conventioneers will be attracted daily, creating a center-city focal point of intense nightpoint of intense nighttime as well as daytime activity. Success for the project would undergird Baltimore's already impressive downtown and neighborhood revitalization.

The message for the outside world could be even more important: that even an old city, burdened with a blue-collar, workaday image and an ample share of the poverty and raacial problems that outsiders say doom older cities, can rebound and turn its center into a vital place for urban people.

Cynics may claim Harborplace represents just another commercial opening, and not an entirely original one at that. It will bear strong resemblance to Boston's Faneuil Hall Marketplace, the highly successful 150-store extravaganza on Boston's waterfront that 15 million visitors annually, also developed by the Rouse Company with Benjamin Thompson as architect.

As Rouse sees it, "In Harborplace, we'll discover whether the excitement of the Faneuil Hall Marketplace reveals the special spirit of Boston or a yearning of people in cities generally for a festival marketplace. If we can prove there's a comparable response -- and I think we will -- then there's a potential for a festival market in every American city, even down to metropolitan areas of a half-million people or less."

But "festival markets" alone cannot a successful city make, as Rouse is quick to note. Indeed, they're unlikely to emerge at all unless other critical elements are present. Business leadership, for example: Baltimore businessmen became alarmed about the city's progressive deterioration in the '50s and then formed the Greater Baltimore Committee, which in turn drew up the two great plans that have "saved" the inner city. The first was for Charles Center, a huge 15-building office and apartment development in the center of downtown, and the second for the Inner Harbor.

On the government side, Baltimore has been fortunate to have a "strong mayor" form of government and a string of able mayors. Since 1971, Mayor William Schaefer has been a near-hero for his dedication to city renewal. The city wisely focused all housing and urban renewal powers in a single housing and community development authority, led for several years by Robert Embry, now U.S. assistant secretary of housing and urban development.

Employing "homesteading" and other innovative housing programs, Embry and his successors have stimulated new construction or rehabilitation for thousands of families of all income groups. They've been skilled at federal "grantsmanship," but government money can't explain the amount of housing restoration now under way. In old Baltimore neighborhoods, it's difficult to find a street where some houses -- often cheek-by-jowl with the ugliest slum structures -- aren't undergoing restoration.

There are some drawbacks: "Gentrification" is forcing poor blacks out of some innercity neighborhoods, causing overcrowding elsewhere in the city. All the downtown efforts have failed to revive the old retail center at Howard and Lexington, where two of the city's four major department stores closed, leaving hulking abandoned buildings. And in such neighborhoods as "Little Italy," there's fear Harborplace will draw of valued restaurant trade.

Harborplace's single greatest virtue is that it "fits" Baltimore's economy and tone of life so well. The city has a well-diversified economic base. But the harbor -- one of the world's largest, receiving over 5,000 ships each year -- literally gave birth to Baltimore, and remains its strongest asset. Around the globe, great harbor cities are known for their color and diversity. Harbor-place will accent those qualities for a city that has not exploited them well for generations.

Other projects already built or planned around the Inner Harbor should build on the same strengths. They range from the 30-story World Trade Center to the Maryland Science Center, from floating restaurants and a new fish market to an aquarium that will include a glass-enclosed Amazon rain forest. And beside Harborplace of the frigate Constellation, the first commissioned warship of the U.s. nAvy.

Thirteen ethnic festivals -- Ukranian, Finnish, Czech, German and others -- are held on the Inner Harbor or at Charles Center each summer. The city's celebration of itself culminates each September with perhaps the grandest urban event in America, the annual Baltimore City Fair. Over 2.5 million city residents and visitors come to hear concerts, to view dozens of neighborhood arts and crafts exhibits and to sample Chesapeake Bay seafood delicacies and ethnic foods.

Developer Rouse hyperbolically claims that "Baltimore in two years will be recognized as the most dazzling center city in America -- in a place that a few years ago wasn't going anywhere." We'll see. But one point remains: ifHarborplace draws people even half as well as its promoters hope, it could transform Baltimore's celebration into a year-round event.