Throughout the nation, old downtowns sparkle anew

Amid brilliant new construction, the facades of many distinguished old buildings now sparkle afresh. Entire blocks in many cities have been or are being restored, rejuvenated, and preserved.

Surprising to many people is that much of the caretaking of the old has become the responsibility of at least a half dozen private nonprofit organizations.

Just recently, one of the groups called Don't Tear It Down (DTID) was one of 18 recipients of a Preservation Honor Award, given annually by the National Trust for Historic Preservation.

DTID has drawn volunteers for more than a decade to implement its programs and has been successful in lobbying for an ordinance which gives buildings on the city's Inventory of Historic Sites and the National Register of Historic Places the strongest legal protection in the country.

Backed by a seed grant from the State Historic Preservation Office, 130 volunteers for Don't Tear It Down surveyed 50 city blocks in Washington's old downtown, collecting information about each building - its age, architect, owners, occupants, usage, and alterations.

The first training program, also conducted by volunteers, resulted in the publication of ''A Guide to Resources for Researching Historic Buildings in Washington, D.C.''

The Joint Committee on Landmarks already has designated a 25-block ''downtown historic district;'' it is, in effect, a preservation plan for the area. It outlines preservation priorities and frees other property for development.

DTID now is working toward landmark designation for each of six sites within the new historic district, including a 1913 office building designed by Appleton P. Clark.

''We consider it an important structure,'' said Alison Kim Hoagland of DTID, ''because it is a unique example of Washington's commercial style. It is only four stories tall but very long, so it looks like a skyscraper on its side.''

The owner of the building is requesting that the designation be limited only to the facade of the building, which is strategically situated at a subway stop where new construction is likely to take place.

Don't Tear It Down is probably most widely recognized for rescuing the city's eclectic Old Post Office building, which is expected to reopen soon as a combined office and shop arcade.

''In several ways, DTID supplants the public sector,'' declares Mrs. Hoagland.

''The process of identifying landmarks is certainly a governmental function, '' she added. ''But in the past decade it has been citizen groups and owners who have submitted all of the landmark applications.''

The goals of the city's preservation groups vary from urging private and commercial owners to place easements on their properties to maintaining the individual historic flavor of neighborhoods.

One of the more effective steps to save a building from zoning changes, the wrecking ball, or even the vagaries of a future owner is an easement which assures that the buiding facade will be retained. What the owner gets out of it is a tax reduction.

Since 1978, the nonprofit L'Enfant Trust has convinced some 55 private and commercial property owners to take this step. The protected structures range from Victorian row houses and a brick apartment building constructed in 1908 to the 1903 brick former townhouse which had been converted to offices and where L'Enfant Trust now is headquartered.

''L'Enfant Trust wants to increase public awareness of neighborhoods other than the monumental areas usually seen by tourists,'' said Margaret S. Dean, executive director.

''We also want to open the eyes of the residents to what is around them.''

A current photo contest, for example, focuses on non-monumental Washington in an attempt to further highlight the city's historic streets and neighborhoods.

There also are active neighborhood groups, among them the DuPont Circle Conservancy and the Foundation for the Preservation of Historic Georgetown.

The DuPont Circle Conservancy was organized 15 years ago to monitor alterations and proposed demolitions in the DuPont Circle and 16th Street Historic Districts, which had just won listings on the National Register of Historic Places.

Among the more dramatic impacts of the conservancy was the rescue of the Demonet Building, slated for demolition. The historic designation was approved and the 1880s structure still stands. To prevent architecturally jarring neighbors, the conservancy actively participated in the critique of an office building that had been proposed for construction behind the Demonet Building.

After two years of researching and readying a 54-page application for extension of the DuPont Circle Historic District, the conservancy now is in the midst of the preliminary hearing process. If approved, the added district will include sites of the home and churches of black leaders, including writers, artists, architects, educators, and a Civil War Medal of Honor winner.

In the quaint and lovely Georgetown section of the city, residents organized the Foundation for the Preservation of Historic Georgetown in 1965 when it became clear that the 1950 Old Georgetown Act, enacted to preserve and protect the character of the district, lacked the teeth required to save historic homes, prevent open spaces from being crowded with new buildings, and halt the intrusion of residential areas by commercial and institutional activities.

The foundation has since been instrumental in restoring the old Georgetown market. It has worked to control the expansion of Georgetown University into residential areas and published and sold 17,000 copies of ''A Walking Guide to Historic Georgetown.''

Easements, urged by the foundation, now permanently protect 61 significant homes with more being negotiated.

''It is through the persistent efforts of the preservation community in Washington, D.C., that we have established the strongest preservation legislation in the country,'' said Michael Ainslie, president of the National Trust for Historic Preservation, chartered by Congress in 1949.