During two weeks next March, Bach specialists will converge on Leipzig, East Germany, to celebrate the 300th anniversary of the composer's birth -- in a series of concerts that will be close to nonstop. Washington's Bach Consort, the only American group to be invited, gave a preview of its widely varied program for the festival last night in a concert at the Kennedy Center.

Last night's benefit was to raise the $100,000 needed to get the Consort's 25 instrumentalists and 40 singers to the city where Bach is buried and reigned supreme during his last 27 years, as kapellmeister at St. Thomas Church. The Leipzig concert, three days after Bach's birth date of March 21, will be the highlight of a two-week European tour, the Consort's second.

In last night's program, the emphasis was on vocal Bach, as it is in the Consort's repertoire.

The grandest moment came at the end, in the festive, proclamative secular cantata "To net ihr Pauken! Erschallet, Trompeten!" ("Strike, ye drums! Sound, ye trumpets"). And that's exactly what the Consort did, along with splendidly precise and pointed choral singing. The solo trumpet, especially in the bass aria, was played with abandon. And massed trumpets soared grandly above the orchestra in the final chorus.

Another exciting moment came in a virtuoso performance on the Concert Hall's Filene Organ by the Consort's music director, J. Reilly Lewis, of the bracing D major Prelude and Fugue.

The program's first half was less stirring. Bach's 71st cantata, "Gott ist mein Ko nig" ("God Is My King"), started off listlessly but gathered force. The A minor Violin Concerto (with Jody Gatwood the soloist) was rhythmically slack in its first two movments, but improved in the last. Bach's Motet for Double Chorus: "Singet dem Herrn ein neus Lied" ("Sing unto the Lord a new song") was well sung.