"Porgy and Bess," perhaps America's grandest opera, is getting an epic production at the Kennedy Center's Opera House. While George Gershwin's jazz masterwork sometimes lumbers like a giant under the direction of Jack O'Brien, it conveys the power and majesty of a giant as well. The current staging, courtesy of Radio City Music Hall and producer Sherwin M. Goldman, is billed as the most fully realized version since the opera's New York premiere in 1935. It is doubtless the most costly. The three-hour extravaganza boasts a score of singers -- plus three different sets of principals for the Washington run -- and 30 jazzy arias, duets and choruses, not counting frequent reprises. There are frame houses that tower three stories over the stage, and a crowded orchestra pit to beat the band. It can be, for novices, occasionally rough going: As one of Gershwin's songs says, "It Takes a Long Pull to Get There." But those who stick it out will be amply rewarded. With a libretto by DuBose Heyward and lyrics by Heyward and Ira Gershwin, the opera focuses on hapless lovers in Charleston, South Carolina of the 1930s. Porgy is a paraplegic who lives in a shack on Catfish Row, the black part of town. Bess is a local tart who moves in with Porgy for a brief spell of happiness. The murderous Crown, Bess' former boyfriend, shows up before long to spoil things -- as does the dandy Sportin' Life, plying her with cocaine and the promise of a better life. The tale pits love against temptation, weakness against strength, and black endurance against white oppression, in the person of Larry Storch as a rumpled detective. It takes on the force of Greek tragedy in Gershwin's gifted hands. The music by now is as classic as anything by Verdi or Puccini, and the conception is equally operatic, but the rhythms and sounds are unmistakably American. Gershwin drew on the Negro spiritual, Tin Pan Alley and his own fertile imagination to compose such songs as "Summertime," "Bess, You Is My Woman Now," and "It Ain't Necessarily So," among other masterpieces. His orchestrations still bubble with originality after all these years; the music still sizzles with the shock of recognition. The music, under the direction of C. William Harwood, is this production's great strength. Radio City has assembled a chorus the likes of which one seldom hears. All convincing soloists separately, these singers together can blow you out of your seat -- and do it with precision and admirable musicianship. The opening-night cast, featuring Michael V. Smartt and Henrietta Elizabeth in the title roles, all sang and acted solidly, though no one really glowed with that undefinable something. George Robert Merritt came closest, with his swaggering, malevolent Crown -- especially in the close of the first act, when he ravishes Bess -- and Gwendolyn Shepherd was a comic treat, flashing a knife at Sportin' Life in Maria's "Struttin' Style." O'Brien's cluttered staging, not helped by George Faison's heavy-footed choreography, worked to make matters drag this first time out in the Opera House. At 11/2 hours, the first act seemed too long. But it was an amazingly trouble-free opening for such a mammoth piece of work. PORGY AND BESS -- At the Opera House through June 18.