"Treasures of Ancient Nigeria," opening today at the Corcoran, is an exhibition of superlatives and revelations.

In this group of 100 bronze and terra cotta sculptures, all on loan from the Nigerian National Museums, Washington audiences can see -- for the first time -- the full two-millennium sweep of African art from 500 B.C. to 1900 A.D. And they can see it not through archaeological fragments, but in masterpieces that rival in beauty, power and craftsmanship the greatest works of art ever produced.

It is no wonder that Warren Robins, director of the co-sponsoring Museum of African Art, has called this "the most important exhibition of African art ever seen in the United States." It will also, he adds, "completely demolish the misconceptions and prejudices long held in the Western world concerning the cultures and peoples of Africa."

That it does. Those who still think African art "primitive" will be astounded by the fabulous twin bronze leopards of Benin, matchless in their dignity and sophistication.

For those who believe that all African art is abstract, there are the subtly rounded, serene and highly naturalistic bronze portrait heads from 15th-century Ife.

And for those whose experience of African art is limited to carved wooden objects made during the past century, the entire exhibition is a resounding surprise. Nothing here is made of wood, and all of the works -- including a stunning bronze ritual vessel of the Igbo-Ukwu culture -- demonstrate the highest technical skill.

Organized by the Detroit Institute of Arts, where it was first shown, the exhibition is arranged chronologically, presenting a succession of pre-20th-century cultural groupings: Nok, Igbo-Ukwu, Ife, Owo, Benin and Tsoede. Each separate unit is handsomely installed and introduced by explanatory material that is remarkably lucid, considering that much of the scholarship surrounding the show is very recent: Though Benin bronzes have been well-known in Europe since the 15th century, the discovery of earlier works came only in this century, often by accident. Digs undertaken by the Nigerian government continue to be some of the great scholarly adventures of our time.

African art seems a somewhat larger subject than can be covered in a show drawn from Nigeria alone. But, for whatever reasons, nine-tenths of all the known ancient art from sub-Saharan Africa has come from that West African nation. In Africa, "ancient art" necessarily refers to sculpture made from clay, stone, bronze or ivory, since wood sculpture -- all that most people know of African art -- survives for only a few decades in the tropical climate.

Thus, when a group of terra cotta heads from 500 B.C. were found in 1943 in the small tin-mining village of Nok, they became the earliest known sculptures in all of sub-Saharan Africa. The most extraordinary of them -- a head dated from 500 B.C. that begins this show -- has a spellbinding presence, simplified forms and Picasso-like, triangular eyes.

This Nok head is one of a dozen or so works on view that traveled to Washington in 1970 for an exhibition of African art at the National Gallery, but it has lost none of its impact. In fact, its exceptional quality is underlined by the presence of other heads from the same discovery. The Nok culture, obviously highly skilled in the art of sculpting clay, continued until 200 A.D.

The first bronzes in this chronology appear in the 9th and 10th century Igbo-Ukwu culture -- the earliest-known workers of copper and its alloys in tropical Africa. With metals probably imported across the Sahara from North Africa, the Igbo-Ukwu artists cast bronze by the lost-wax method, developing a highly ornate, decorative style that was lavished on objects used in royal and religious ceremonies.

On view here are several ritual objects accidentally found in 1938: pots and bowls, including an elongated water pot enclosed by a rope-like cage that represents virtuoso casting. Several other works here take the form of carved calabash gourds and are covered with tiny spirals, abstract flowers and, occasionally, a true-to-life insect form.

It is in the next segment of the show -- dealing with the art of the city-state of Ife from the 12th to 15th centuries -- that the loudest exclamations will likely be heard from viewers. And rightly so: Here, from a continent where we have been taught to expect only simplified, abstracted human forms, comes a unique group of life-size and stunningly lifelike bronze and terra cotta portrait heads of dead kings, or Oni. Of them all, "Crowned Head of an Oni" is likely to rivet our gaze longest, so serene and regal, so timeless and inscrutable is her face. oThese exquisite bronzes are reminiscent of the sculpted head of the Egyptian Princess Nefertiti and Leonardo's Ginevra de'Benci. Equally fine are the "Head of a Queen," "The Usurper Lajuwa" and "Face with Cat's Whisker Marks," rendered in terra cotta.

In the next culture in the exhibition -- 15th-century Owo -- sacrifice is a recurring theme. These terra cotta heads and fragments of hands making offerings show Ife influence, but little of their finesse.

When Ife political power began to wane in the 16th century, Owo fell under the domination of the growing Benin empire, and the show draws to a close with the relatively late sculptural achievement of the wealthy Benin kings from the 15th to the 19th centuries. Ironically, Benin has been perceived by many laymen until now as the earliest -- and the only -- great and lasting sculptural achievement in tropical Africa.

That it is one of the greatest, however, is surely underlined here, not only in the familiar "Queen Mother" bust (also shown at the National Gallery 10 years ago), but in an exquisite bronze stool incised with the forms of intertwined mudfish, and in the pair of fearsome and regal 16th-century leopards. They were put on sale in New York in the 1930s at $15,000 each, and remained unsold.

The leopards -- and every other Benin object in this exhibition -- had to be purchased by the nigerian government. Along with more than 2,000 other objects, they were taken from Benin by the British during a punitive raid in 1897. They were subsequently dispersed and sold to museums all over the world.

The show -- sponsored by Mobil Oil -- continues through January. A related program of lectures, films, workshops and performances begins this Sunday at the Corcoran. For information, call the Corcoran Education Department at 638-3211.