Some 500 persons involved in the revitalization of America's older cities -- architects, planners, city officials, lenders, investors, syndicators, engineers, contractors and real estate executives -- came together for three days here to discuss the deep problems and great opportunities of their efforts.

The cities are "one of our richest and most significant natural resources," said Chicago developer Thomas J. Klutznick. And, declared Philadelphia architect Richard W. Huffman, offer "great investment opportunities."

But they and other speakers made clear that unlike the city-dominated urban renewal programs of the 1950s and 1960s the participation of every portion of both the private and public sectors -- with everyone sharing in the risks as well as the rewards -- was necessary to make revitalization work.

"You look at those urban renewal projects and you wonder what got renewed," commented M. J. (Jay) Brodie, Baltimore's housing and community development commissioner, who compared the strictly delineated public and private roles of the urban renewal period with the joint planning between his city and private developers for Harborplace and other redevelopment projects.

"Being regulatory and custodial is not enough for the public sector," said Leo Molinaro, president of American City Corp., the Rouse Company subsidiary that is currently involved in downtown and waterfront redevelopment projects in 25 cities. "Public partnership is the only way to go. Cities have to take an entrepreneurial view. The acceptance of risk on both sides makes it a true partnership."

It was in this new cooperative atmosphere that these diverse but interrelated professions were gathered at the first City Recon Congress, which was sponsored by the National Association of Housing and Redevelopment Officials and supported by 22 other national organizations.

"This is the strongest group of authorities in the field I have ever seen at one conference," commented George Sternlieb, director of the Center for Urban Policy Research at Rutgers University, during a discussion of "Opportunities in Urban Rebuilding."

Projects also can't exist in isolation if urban revitalization efforts are to succeed was another message presented by speakers. City plans will be more comprehensive and "reflect a greater degree of attention to planning issues than to architectural issues," said Klutznick, developer of Chicago's successful highrise shopping center, Water Tower Place. "Buildings can no longer be designed in the abstract, ignoring their impact on the surrounding quality of life."

Sternlieb stressed that three things are needed for successful downtown revitalization -- office workers, a residential population with buying power and a unique ambience to pull in persons from outside the area.

"There are few locations that can secure all three," he said. "If you can't produce at least two of them, you can't create retailing."

Although Sternlieb did not mention Baltimore, other speakers made clear that city was successful in meeting these requirements with its Harborplace and adjacent developments.

"Harborplace is not just a retail market area, it's a major place for regional activities," said Huffman.

As much as Baltimore's successes captured the minds of attendees, the fact that those successes were achieved without tax abatements seemed even more amazing.

Buffalo also tries not to give tax abatements, said Lawrence Quinn, community development commissioner for the upstate New York municipality which also enters into partnerships with developers. "We do equity partnerships," he explained. "The city gives a minimum interest rate, but takes a piece of the deal. The returns from all these projects are replacing the federal money we're losing."

The loss of the once important participation of the federal government was one of the problems discussed at the conference. Klutznick charged that the Reagan administration's supply-side theories which are supposed to result in windfalls for cities are "sheer hogwash."

"There is no way that supply side economics can sustain the redevelopment of cities," he declared.

However, Donald I. Hovde, undersecretary of the Department of Housing and Urban Development said he is "more optimistic now than at any time in my 29 years in the housing industry" because of improving economic conditions. He challenged his audience to take "advantage of the platter of opportunities that lies before you."

Downbeat notes were struck by other speakers who placed revitalization efforts in the context of what University of Chicago demographer Philip Hauser referred to as "the emergence of an underclass in our cities with no prospect of being eliminated."

"Revitalization is for people who aren't poor," said architect Huffman.

"We're now seeing," said urbanologist Sternlieb, "the development of the city within a city phenomenon." Unlike the downtowns of the past with their large department stores, revitalized downtowns no longer offer "something for everyone," he explained.

It is in the "oldness" of buildings that cities now have strengths rather than weaknesses, although an aging, deteriorating infrastructure including roads, bridges and public transportation presents major problems, noted Hauser.

"There are more opportunities for construction booms through upgrading, than in new construction," said Hauser.

Similarly, Chicago developer Lawrence Levy said he finds that because of their "beautiful stock of buildings . . .the greatest opportunities for revitalization will be in those cities people have written off, such as Cleveland and Detroit."