There will be a standing-room-only audience tonight in the Kennedy Center Concert Hall. That happens every year when the biggest and most enthusiastic chorus in Washington (always a hotbed of choral singing) gets together to sing Handel's "Messiah," demands an encore of the "Hallelujah" chorus and gives itself a standing ovation.

The audience loves the chorus every year at the Kennedy Center's "Messiah" sing-along. That's because the audience is the chorus.

If you search around, it is possible to find as many as half a dozen "Messiah" sing-alongs in the Washington area around Christmas time -- and there are probably some eager singers who go to all of them. But the big one is tonight's performance at the Kennedy Center. The tickets have been sold out since a few hours after they became available. Strictly speaking, they were given out, because they are free -- unlike tickets for "Messiah" sing-alongs in the major concert halls of such cities as Chicago, Cleveland, Pittsburgh and New York, where tickets for a similar event last year at Avery Fisher Hall ran from $12 to $50.

People begin to line up at the Kennedy Center about 5 a.m. on ticket day, bringing blankets, lunches and decks of cards like rock fans lining up outside a stadium. But there usually are 700 to 800 no-shows on sing-along night by the time the tickets expire, at 15 minutes before the performance begins, so it may be worthwhile to arrive and wait in the foyer, with a score, in hopes of grabbing an unclaimed seat at the last minute.

There will be a regular chorus on the Concert Hall stage -- the Paul Hill Chorale, whose music director will be the master of ceremonies and one of the evening's conductors. Apparently, the "Messiah" sing-along at the Kennedy Center is such a strenuous job that several conductors are needed -- or perhaps there is so much glory in the occasion that it has to be shared.

Whatever the reason, Hill will be relieved from time to time by several associate conductors. Two of them rank among the nation's leading choral directors: Tamara Brooks, music director of the Mendelssohn Club of Philadelphia, and Edward Polochick, director of choral activities for the Baltimore Symphony and the Peabody Conservatory.

The third will be Martin Feinstein, general director of the Washington Opera, who has conducted the "Hallelujah" Chorus regularly since the sing-alongs began at the Kennedy Center -- and who might become violent if the assignment were taken away from him. There are some who claim he gets violent anyway.

"Martin conducts the fastest 'Hallelujah' Chorus I have ever experienced," says one veteran Washington choral singer, "and everybody goes crazy about it." The audience always demands an encore, and emcee Hill usually allows it if there is no danger of running into overtime pay for the free-lance orchestra. Maybe the looming threat of overtime explains Feinstein's tempos.

"Messiah" has had a special place in the hearts of audiences and singers alike for more than two centuries. You can hear it when the whole audience is singing "For unto us a child is born" or "All we like sheep have gone astray." And you can see it at concerts where the audience comes just to listen.

You can even see it during intermissions. Those at the National Symphony's recent series of "Messiah" performances differed subtly from most intermissions at the Kennedy Center. Out in the foyer, as always, were the little vendors' stands operated by volunteers from the Friends of the Kennedy Center. To raise money for special, free programs, these volunteers will sell patrons everything from chocolate mints to T-shirts. Librettos and opera glasses are hot items at opera intermissions -- and occasionally records are sold when a popular soloist or group is on stage inside.

But the fastest-selling item at "Messiah" intermissions is none of the above -- it is the music itself, a piano-vocal score of Handel's dearly beloved oratorio.

This makes "Messiah" different from all the other music that is performed in Washington. Nobody goes browsing at intermission for the sheet music of Verdi's "Requiem" or Carl Orff's "Carmina Burana" when these popular works are being performed in the Concert Hall. And nobody -- or at least almost nobody -- reads along with the music at most performances. Occasionally you may see a young tenor following a world-class model such as Peter Schreier in a Schubert Lieder recital and jotting down notes on his own copy of the music. Or a conscientious music critic trying to juggle score and notebook while he checks to see if anything has been cut from a lengthy masterpiece -- particularly an unfamiliar masterpiece such as Schoenberg's "Moses und Aron."

At a "Messiah" performance, the audience is often liberally sprinkled with score-readers, and the management is likely to get complaints if the lights are turned down so low that reading is difficult. What we have here is unique in our concert life. It means, in part, that some members of the audience want to get as close to the music as possible, using their eyes as well as their ears to take it all in. But mostly the readers are closet choristers who use the performance as a sort of silent rehearsal for their own big moment in a "Messiah" sing-along.