No wonder the Kennedy Center has trouble with the roof -- the "Messiah" Sing-Along last night just about blew if off.

For the ninth year, not only was the Concert Hall packed for this most popular of all events at the Center, but at least 400 more people stood in the giant lobby to belt out some of the most glorious music ever written.

With the Paul Hill Chorale and an orchestra provided by a performance trust fund of the Musicians Union (Local 161-710), with perhaps 2,000 members of the audience brandishing scores and a series of leaders offering advice, it just couldn't miss.

They say that Handel wept when he wrote this oratorio, so overcome was he by the sheer beauty of it all. Last night's performance did him proud, though it was interrupted now and then to clarify an attack or remind the Sunday singers to "move right along on those notes with the dots over 'em."

It had all the minor irritations of a rehersal -- and also that raw vitality.

The free tickets were handed out Saturday starting at noon. By 2 P.M. they were all gone. The line for no-shows began forming two hours before the performance. And when the nonholders of tickets were admitted, they flowed down the aisles and right up onto the stage at Hill's invitation.

"Plenty of room for everybody," he called out. In a moment the stage was SRO: down jackets rubbing shoulders with dinner jackets.

Led by the soloists, soprano Doralene Davis, mezzo Rose Taylor (who sang the work a year ago, three hours after her wedding) and bass Donald Miller, plus a succession of conductors who came on like pitchers at an all-star game -- Norman Scribner, Barry Hemphill and the Center's own Martin Feinstein -- the huge crowd sang like pros.

Most of those who didn't have the score stood up anyway during the choruses.

A few didn't need scores. A lady in a cherry-red dress stood at the railing of the very top balcony, head up, letting it go.

"When you get to those high G's, sopranos," warned Scribner, "you gotta lead us. I'm afraid the burden of this piece is on you."

They came through just fine, and then applauded themselves. In fact, we all applauded ourselves quite a bit.

There were one or two hisses when Hill announced that the "Hallelujah" chorus would be placed at the end of the piece, after the great "Worthy Is the Lamb" chorus and indeed after the "Amen," which has got to be the amen of all amens.

But it worked. It worked so well that they did it twice. Everyone was up for this one, even the nonsingers, and for a few minutes all you could hear was this marvelous shout, everyone together, for once blowing away like leaves in a gale all the rude clerks and paper rustlers and furious drivers and mindless snatchers and elbowers of the marketplace. For a few minutes it really felt like Christmas.