When a new shopping center opened here Aug. 9, this city held a parade, hung banners at street corners, announced the event with trumpeters and hired a daredevil to walk a tightrope above the opening-day crowd.

And now, the city officials and developers who spent more than $170 million to build the center in a previously decaying business district populated by derelicts and illuminated by the neon glow of strip joints may be walking an economic tightrope.

As the speechmakers noted repeatedly during the festivities marking the opening of the center, called Horton Plaza, most San Diego shoppers probably had not visited the neighborhood, referred to as ''south of Broadway,'' for years. Like many Americans, residents here shop at suburban malls.

But there was little acknowledged skepticism about the success of the project amid the civic jubilation. It was hailed as the prototype of a new kind of urban shopping district that would help shape the future of not only this city but others as well.