THE SIDES have already been drawn: Is it, as one critic has said, a "spectacular" building of "imperial" quality, or does it fit the description given it by an angry citizen in a letter to The Washington Post - "the ugliest building in all history"?

It is not surprising that the National Gallery of Art's new East Building should have elicited such hyperboles months before its June 1 opening. Designed by the eminent, some might say preeminent, American architect I. M. Pei, the modern structure was erected next to the Gallery's original neoclassical edifice at Pennsylvania avenue and 4th streets NW at a cost of $94.4 million. It could be called, perhaps, the National Gallery's most lavish and expensive work of art to date.

If so, this latest work of art must be measured by a peerless standard, that set by the donation which began the Gallery's Permanent Collection - Andrew Mellon's original gift of 125 masterpieces by such artists as Boticelli, Jan van Eyck, Raphael, Titian, Rembrandt and Rubens. In fact, the Joint Resolution of Congress establishing the National Gallery of Art in 1937 carried this charge: "No work of art shall be included in the Permanent Collection unless it be of a similar high standard of quality to those acquired from the donor." Forty years and 40,000 acquisitions later, some will wonder if the East Building measures up.

Few are likely to be neutral. Most, however, even those now offended by the massiveness of the exterior, will probably be seduced by the interior of the building once they submit themselves to it. When the members of Congress were invited over one recent, afternoon for champagne and a sneak preview, "they loved it," according to Gallery director, J. Carter Brown. The ultimate test will come when Congress decides whether to appropriate the requested $16 million, including supplements, for operating costs in Fiscal 1978, and the $19 million requested for Fiscal 1979, up from $12,686,000 in 1977.

The National Gallery East Building crowns a decade in which palaces of culture have proliferated at a phenomenal rate in Washington: the National Collection of Fine Arts and the National Portrait Gallery in 1968, the Kennedy Center in 1971, the Renwick in 1972, the $16 million Hirshhorn in 1974, the $34 million National Air and Space Museum in 1976. These, added to the already existing museums along the Mall, might well lead historians of the future to look back on this era as the Golden Age of Culture Worship and on Washington as the Athens-on-the-Potomac.

For them, it will probably be even clearer that the new East Building is a qualitative capstone as well and one of the few great twentieth century public buildings in Washington, surpassed only by Dulles Airport and rivaled only by the felicitous facade of the Air and Space Museum. It is clearly a work of art. How great, only use and time can decide.

An immediate surprise awaits first-time visitors: the discovery that the "new building" is, in fact, not one but two triangular buildings set in a trapezoidal site from which their complex geometry derives. The site, set aside for Gallery expansion back in 1937, has been occupied in the interim by simple rectangles in which people played tennis.

The bold "H" of the 4th street facade unites the buildings on the outside; on the inside they are joined by the massive and magnificent faceted glass skylight, called a "space frame," which rises to 107 feet, nesting Alexander Calder's grand three-story mobile.

Directly behind the "H" of the facade lies the exhibition building, the public part of the facility. The "towers" hold three levels of "pod" galleries, intimate carpeted spaces for exhibitions of small-scale drawings, prints and paintings. The horizontal connections between the towers define larger gallery spaces on the upper level where temporary exhibitions will be held, and where part of the Gallery's twentieth century collections will be displayed on a continuing basis.

"We prefer 'continuing' to 'permanent' at this point," says David Scott, the Gallery's planning consultant and former director of the NCFA, who has supervised the project since 1969. "Some of the exhibitions will be more 'continuing' than others." Plans for the near future include exhibitions of Fragonard and Edvard Munch, among others.

Visible only from the Mall and Capitol sides, the second building in the complex, the Center fro Advanced Study in the Visual Arts, rises to eight-story grandeur. When it opens in 1979, it will fulfill a dream of Bernard Berenson, "who wanted Washington to be a center of scholarship for our age such as the library of Alexandria (Egypt), burnt in A.D. 391, had been for Hellenistic times," according to John Walker, Brown's predecessor, who shared the dream and initiated the whole concept of the new building before he retired in 1969. The National Gallery tends to think big.

The entrance to the Study Center is behind the dramatic knife-edge at the extreme right of the 4th street facade, and has at its core another vertiginous six-story reading room surrounded by book stacks and balconies where National Gallery staff as well as visiting scholars will ultimately be housed in parallelogram-shaped (and often windowless) offices. The building will also house the Gallery's growing library and photo archives, recently expanded by a gift of 20,000 photographs from the Federal Republic of Germany.

At penthouse level, overlooking the most spectacular view of the Capitol in Washington, perch the offices of Brown and Paul Mellon, the president of the Board of Trustees, who, with his late sister Ailsa Mellon Bruce, paid the entire $94.4 million required to build the East Building complex. It was their father, Andrew Mellon, who built the original National Gallery in 1941 for $15 million.

The simplest way to enter the East Building is from 4th street, into a low-ceilinged reception area which suddenly reveals - the spectacular skylit courtyard.

The brave and hearty can also enter from the original building, via the new underground connecting link, through the ear-shattering din of the Cafe/Buffet. (Intended to recall sipping Pernod in Paris, the Cafe/Buffet experience is more like drinking beer in the middle of the Beltway during rush hour.) Once you have overcome that obstacle, however, a moving sidewalk carries you to the lower concourse of the new building, landing in front of a large tapestry designed by Jean Arp, right next to the temporary exhibition area where "The Splendor of Dresden" has been installed, just past two jousting knights in armor. The Gallery's two new auditoria, the larger of which seast 400, are also entered from this point.

But the first-time visitor should proceed up the steps to see the central court. It is here that the experience of the building begins: the vast, exhilarating skylit space dominated by the largest and most wonderful Calder in the world, 980 pounds of aluminum moving like a down feather in a gentle breeze; and the huge one-ton-plus Joan Miro tapestry which dominates the south wall, dwarfing four sixteen-foot ficus trees.

A commissioned steel piece by contemporary sculptor Anthony Caro, perched on a ledge over the door to the Study Center, gradually comes into view, looking at first glance - and possibly longer - like something the construction crew left behind. And if fro some the space increasingly takes on the look and feel of a holy place - a sort of Cathedral of Art - surely the timeless and immutable stone sculpture by Noguchi seems a likely candidate for high altar.

A Maillol "Venus," goddess of love and beauty, stands nearby. She'd have loved the classic geometries of the place: isosceles triangles in the space frame above and in the marble floor below, combining to make parallelograms of the towers, which, cut off at the corners, make bexagons of the galleries - and the elevators.

The building is a delight for the mind as well as the eye, a fact which delivers it firmly into the realm of high art. Whether it proves to be a delight for those people who have to work in parallelograms, howevers, remains to be seen.