If you've been taking your time getting tickets to performances of Gustav Mahler's colossal Eighth Symphony -- more famously known as the "Symphony of a Thousand" -- you're out of luck: Seats for all three of the National Symphony Orchestra's performances this week have been sold out for nearly a month.

"It's a spectacle -- it's larger than life," says NSO Music Director Leonard Slatkin, explaining how the famously neurotic Mahler has become the hottest ticket in town. "Because of its sheer size, it just isn't done very often -- and for a lot of people, this will be a once-in-a-lifetime experience."

But even Slatkin and company have been caught off guard by the interest in the work, which opens tonight at the Kennedy Center. For the past few weeks, they've been furiously pulling the massive piece together, and while there won't be literally a thousand performers onstage (the nickname was a gimmick cooked up by an early impresario), the assembled forces are still staggering: eight soloists, an adult chorus of 314 singers, a children's chorus of 60 and a beefed-up orchestra of about 120 (including multiple harps, a pianist and at least one mandolin), for a grand total of roughly 500 musicians -- one for every five members of the audience.

All those performers, meanwhile, are being arrayed throughout the Concert Hall, from the onstage risers to the chorister boxes and up into the first tier, and the stage itself has been extended a full 20 feet into the audience -- the better to unleash a sonic tsunami that hasn't been heard here since 1988.

"There's no wilder ride than the Mahler Eighth!" says Robert Shafer, sounding partly elated and partly terrified. The Washington Chorus music director has been in rehearsals since March, going over a complex score in which as many as 24 different parts are sung simultaneously. "And in this performance the big parts are going to be incredible," he adds, "because the audience will be surrounded with the music."

But make no mistake -- the music, not the sheer numbers, is the real draw. Passionate, transcendental and unabashedly joyous, the Eighth is a full-throttle affirmation of life -- on a par, many believe, with Beethoven's Ninth. Mahler himself certainly thought so. "It is something the world has never heard the likes of before," he wrote to a friend in 1906, shortly after finishing the work. "Imagine the universe beginning to ring and resound. There are no longer human voices, but planets and suns revolving in their orbits."

Megalomaniacal? Not really, says Mahler biographer Henry-Louis de La Grange, who has spent more years studying Mahler's life than Mahler spent living it. The splendor of the Eighth, La Grange says, embodies an optimistic side of the composer that was almost never seen -- but is just as real as his more notorious dark side. "He was in a state of real ecstasy when he wrote that symphony," says La Grange in a telephone interview from his home in Switzerland. "And it's a great spiritual statement -- a message of hope for humanity as a whole. Some people find it hard to understand, because they feel that Mahler has to be morbid or he isn't Mahler -- but that is just simply wrong!"