Once these neighborhoods were the homes of the city's upward mobile. Then came the inevitable aging, the rush to the suburbs in the post-World War II years and the resultant change and decline.

Now these decayed neighborhoods in New York City are symbols of the troubles of the graying inner cities of America -- Bedford-Stuyvesant, South Bronx, Brownsville, Central Harlem, South Jamaica, Bushwick, Coney Island, East New York and Prospect Heights.

An ambitious home-ownership plan for low- and moderate-income New York City families, aided by a federal mortgage subsidy program, offers hope for a return to the past glories of the old neighborhoods.

For as little as $1,368 down, a working person can buy a home with a $45,600 price tag and a 4 percent mortgage in what amounts to a 1980s homesteading program, it was announced by Mayor Edward I. Koch and Anthony Gliedman, commissioner of Housing Preservation and Development.

The average cost of a new home in the nation is around $80,000 with mortgage rates in the neighborhood of 15 percent and down payments of a quarter to a third of the sale price.

Two thousand houses will be sold in 10 rundown city neighborhoods, with prices ranging from $45,600 to $52,800, at interest rates of 4 to 10 percent. Most of the homes are expected to be ready for occupancy in the spring of 1982.

Under a federal interest subsidy program, qualified buyers can have their mortgage interest rates reduced to as low as 4 percent.

Koch, who campaigned on the theme that he wanted to preside over the renaissance of the city, announced the program while standing on a site at Columbia and President streets in the dilapidated Red Hook section of Brooklyn.

"Last June we stood on Columbia Street and issued a friendly challenge to private developers to come up with plans to construct affordable homes for New Yorkers," the mayor said. "I'm pleased to say today that they rose to that challenge."

Gliedman said, "Turning tenants into owners, as we are doing on Columbia Street and elsewhere, gives them a stake in the neighborhood and is one important step in solving the city's housing crisis."

A portrait of the way things used to be was apparently used as a model for the Columbia Street development.

The homes will have stoops, landscaped seating areas and street parking. The block will resemble a row of traditional Brooklyn townhouses.

Of the 203 homes on Columbia Street, 173 will be newly constructed and 30 will be rehabilitated existing structures, according to Gliedman. The new construction will include some deluxe apartments in the three-story condominiums.

The area will also contain new stores.

"With local and federal officials working together," Gliedman said, "we can transform neighborhoods all over this area, this borough and this city."