For more than 100 years, opera audiences never heard Modest Mussorgsky's original version of "Boris Gudunov." His contemporaries found the 1869 composition too awkward, rough-hewn and even dissonant, so several composers tried to "fix" the score themselves.

Today, the opera is being heard once again in its original form, and its dark musical richness is a feast for the ears. San Diego Opera debuted a new production of "Boris" on Saturday featuring the 1869 score, and while its theatrical structure is admittedly odd, it's a sweeping, epic and very Russian piece of music that deserves to be heard in its original.

The viola-rich score, conducted with vibrant energy by Valery Ryvkin, is packed with emotionally charged passages and folk-style melodies, as well as grand choral numbers with multilayered harmonies that ebb and flow with a liquid grace.

And it is the 94-member chorus, trained impeccably by chorus master Timothy Todd Simmons, that represents a key figure in Mussorgsky's masterpiece -- the Russian people. The opera's weight is counterbalanced between the suffering of the peasants and the guilt-plagued ambitions of the czar, Boris Gudunov.

Starring in the production as Boris is the world-renowned Italian bass Ferruccio Furlanetto, who is as fine an actor as he is a singer, and that's saying a lot. Furlanetto is the only Italian to have ever sung the role of Boris in Russia, and it's a role that will become his signature. He imbues the role with a weighty grief that grows into madness followed by a fearless death scene that most opera singers wouldn't attempt. And his huge, resonant bass voice is in perfect control but ripples with pain and emotion. Furlanetto has been seen in San Diego in several roles, but never has he commanded the stage more impressively than as Boris.

"Boris Gudunov" is the story of a 16th-century Russian czar who's plagued with guilt over the devious way he ascended to power -- by the murder of Dmitri, the 7-year-old heir to the throne. Years later, a pretender to the throne (an ambitious ex-monk named Grigori who calls himself the real Dmitri) gathers troops in Lithuania and marches to Moscow to challenge Boris. The combined pressures of his guilt and the specter of Dmitri raised from the dead drives Boris into a fatal epileptic seizure.

All of these details are good to know before the opera begins because Mussorgsky's libretto is choppy and a bit hard to follow. Grigori disappears after the first act and many of the other central characters -- like the devious Prince Shuisky and the Duma leader Tchelkalov -- aren't identified or sketched in very well. A thorough reading of the synopsis in the program beforehand is a huge help to figuring out who's who. Fortunately, director Lotfi Mansouri has worked to simplify and clarify the action, and he should be lauded for his eye-popping staging of the large-scale choral scenes.

Another unusual aspect of the opera is its dramatic structure. Written as seven individual scenes (with set changes in between), the opera's momentum is continually interrupted and several scenes end anticlimactically. The intention here is for dramatic effect, rather than musical effect. Italian opera fans accustomed to scenes with big ensemble numbers and finales may be put off by the style, but they will still be drawn in by the gorgeous music and especially the epic chorus scenes.

Vocally, the opera is unique with only a few small women's roles and a male cast made up entirely of basses and tenors (no baritones). The tone of bass and tenor voices can easily cut through large orchestras, as they do here, and San Diego Opera general director Ian Campbell has found an excellent and balanced cast to fill these roles.

Standouts include Jay Hunter Morris as Grigori/false Dmitri. Although his acting is low-key, his bell-like tenor voice pings with intensity. Russian bass Mikhail Svetlov delights as the drunken monk Varlaam. He has a sonorous voice and comedic skills that are welcome in this mostly dark opera.

Vitalij Kowaljow, a bass from Ukraine, has a smooth vocal line and powerful delivery as Pimen, the aged monk whose chronicle of Boris' rise to power inspires Grigori to challenge the Czar's power. Doug Jones' boyish tenor voice ideally fits the small but critical role of the feeble-minded Simpleton (the only Muscovite brave enough to confront Boris about the murder). And the role of Prince Shuisky is played by tenor Allan Glassman (who, interestingly, played Grigori/Dmitri in San Diego Opera's last production of "Boris" in 1989).

Rounding out the large principal cast are mezzo-soprano Lisa Agazzi in the pants role of Boris' young son, Feodor; the hilarious mezzo Judith Christin as the Inn hostess; Soprano Inna Dukach as Boris' daughter, Xenia; Joseph Frank as the comic monk Missail; Martha Jane Howe as the old nurse; Nicola Janitzky as Tchelkalov and Louis Lebherz as Pristav, the border guard.

The opera's physical production is as impressive as its cast. The sets, owned by Seattle Opera, are simple but effective and can be changed quickly during the multiple scene changes. A large painted curtain of religious icons has a wonderful foreboding effect. With light shining behind the curtain, the iconic saints appear to be staring down on Boris in eternal judgment. And the production's more than 240 costumes are lavish to the extreme, with fur-lined robes, pearl-encrusted fabrics, elaborate hats and more.

"Boris Gudunov" is a sweeping epic of an opera that's a promising beginning for San Diego Opera's 2007 season. Next up in February is another epic, Camille Saint-Saens' "Samson and Delilah" starring Denyce Graves.

"Boris Gudunov" runs two hours, 50 minutes, with one intermission.