IT WAS THE MOMENT Bay Area music lovers had been eagerly awaiting all season. With Thursday night's opening of "Mlada," Michael Tilson Thomas and the San Francisco Symphony unveiled a spectacular performance of Rimsky-Korsakov's seldom-staged fantasy opera-ballet.

Scintillating and suitably large-scale, the composer's four-act extravaganza brings the Symphony's three-week Russian Festival to an impressive conclusion. The final performances, tonight at 8 and Sunday at 7 p.m., represent what may amount to a once-in-a-lifetime opportunity to witness this unusual and endlessly fascinating work.

Thursday's opening, a S.F. Symphony premiere, made it easy to see why "Mlada" is rarely performed. Composed in 1890, it calls for a massive orchestra that includes harps, organ, pan pipes and expanded wind, brass and percussion sections, as well as a large mixed chorus and an extensive contingent of vocal soloists. And, because it is ballet as well as opera, the title role is danced by a ballerina, rather than sung by a vocalist.

The scoring is brilliantly original, as well as influential -- composers including Debussy, Prokofiev and Stravinsky borrowed from it in the years that followed its 1892 premiere at St. Petersburg's Mariinsky Theatre. Under Tilson Thomas' direction, it sounded both familiar and wonderfully fresh.

Often compared to Wagner's "Ring" cycle, the work is based on ancient Slavic myths and legends. Like the "Ring" operas, "Mlada" pits good vs. evil, love vs. greed and envy. The story begins with the murder of Princess Mlada by a jealous rival, Voislava. Mlada's fiancee, Prince Yaromir, spends the rest of the opera seeking revenge. With stops along the way at peasant festivals, temples of the gods, a witches' sabbath and a frightening underworld, it's a colorful journey. In the apocalyptic final scene, reminiscent of "Gotterdammerung," Mlada and Yaromir survive tempests and floods to emerge unscathed, united in love.

Any production of the work is a mammoth undertaking, but all was in place for Thursday's performance. Evelyn Cisneros danced the title role, choreographed by Val Caniparoli, with the poise and fluidity that characterized her 20-year tenure as principal dancer of the San Francisco Ballet.

The vocal soloists were equally first-rate. Soprano Ljuba Kazarnovskaya brought large, gleaming tone to the role of Voislava. Tenor Gegam Grigorian sang heroically as Yaromir. Bass Tigran Martirossian was a resonant Prince Mstivoy, and mezzo-soprano Susanna Poretsky sang attractively in the dual role of Sviatokhna/Morena. As Lumir, countertenor Brian Asawa's pure tone and excellent projection were decided assets.

Vladmir Glushchak, Philip Skinner, Susan Narucki, Datevig Yaralian, David Peters, Corey Head, Kevin Gibbs and Mark Sullivan made strong contributions in supporting roles. Vance George's Symphony Chorus, singing with fearsome dynamics and precise idiomatic phrasing, sounded magnificent.

Peter McClintock's effective concert staging seemed to fill the hall. Much of the action unfolded on two raised platforms above the stage, with performers making entrances around the orchestra and through the audience. In several scenes, singers and offstage bands consisting of horns and pan pipes appeared in upper terraces. Sandra J. Woodall's set, a blue-and-gold expanse of sun and sky, was dramatically lit by Jack Carpenter.

At the center of it all was Tilson Thomas, leading the orchestra through the two-hour, 40-minute score's myriad dances, marches, folk melodies, choral outbursts and extended orchestral passages. With this conductor on the podium, "Mlada" cast a magical spell.