THE Pennsylvania-born organist Cameron Carpenter, recently turned 30 and an alumnus of the Juilliard School in New York, gave two afternoon recitals in the late stages of the BBC Proms, writes Roderic Dunnett.

Carpenter is certainly a showman. His treatment of Bach (a recent Bach recording is entitled Cameron Live!) is flashy, springy, eager, not to say idiosyncratic, full of artful decoration and unexpected delayed or skipping, skedaddling cadences. In this he recalls - and indeed respects - the great organists of the late 19th century, never afraid to add a virtuosic touch.

But Carpenter is patently a great organist, thrilling to listen to and equally to watch. He plays usually without music, revealing a phenomenal memory, not just for the notes, but for his astoundingly versatile, dazzling, and often astute (if just occasionally perverse) shifts in registration. For a Bach prelude and fugue, such as the great F-major with which he began his twin appearances at the Royal Albert Hall, eschewing as dull the preference for a single-registration treatment, he is liable to engage all four manuals, and widely differing timbres and dynamics.

His pedalling technique - witness his arrangement of a Bach G-major Prelude and Fugue from The Well Tempered Clavier - is amazingly fluent and, in places, flamboyant. He loves the instrument, and has had an affinity with it, he says, since the age of four.

His gift for both arranging and improvising was apparent from his treatment of a Bourrée from Bach's C-major Cello Suite, an approach so spirited, personal, and alluring that it recalled for me the expressive, alluring music of post-war French film, the Nouvelle Vague.

Carpenter has a penchant for Busoni, displayed in a daring (though more recognisable than expected) approach to Bach's D-minor Toccata and Fugue, and to his Chorale Prelude on "Nun freut euch, lieben Christen", in which Busoni's famed piano transcriptions form a starting point, or at least a thick-layered aide to the voyage.

He has a flair for Bach's solo-violin Partitas: extracts from no. 3 in E major and no. 2 in D minor (the Chaconne that Busoni famously transcribed, and that Brahms compressed into a version for piano left hand) both figured. The latter was concluded by his arrangement of the finale of Mahler's Fifth Symphony: Cameron's versatility clearly knows no bounds.

Britain has organ soloists of comparable flair and innovation: David Briggs, no stranger to improvisation and transcription, is one scintillating example; Wayne Marshall is another. But Carpenter is a one-off: a kind of Nigel Kennedy of the organ. One's only reservation was a slight rhythmic casualness or looseness in places where none was intended. Ever inventive and garishly bold, Carpenter can get away with almost anything, slickly and slyly galvanising his audience into thinking afresh. Only the odd lax or sleazy blip seemed to detract.

THE conductor Riccardo Chailly and his Gewandhaus Orchestra from Leipzig - the outstanding ensemble that Mendelssohn directed - offered an all-Mendelssohn evening, rounded off by his "Reformation" Symphony (see above); but Chailly proved his real mettle at the orchestra's second Prom.

Here, Mahler's Sixth Symphony followed a landmark work by the deeply religious organist-composer Olivier Messiaen: Et exspecto resurrectionen mortuorum (1964), nominally another anniversary tribute, here to the dead of both world wars, recalling 1914 - the onset of the Great War - and 1944, the summer of France's liberation.

The Messiaen is a mesmerising work, employing not a choir, but weighty wind, brass, and an awesome percussion array - gongs, woodblocks, bells, tam-tams, etc. - to evoke, in five meaty sections, passages from Psalm 130 ("Out of the depths"), Romans, St John's Gospel, and 1 Corinthians 15, followed by the book of Job and Revelation 2 ("They shall be raised in glory with a new name - with the morning stars singing together and all the sons of God shouting for joy"); and concluding with Revelation 19.

Thickly scored, Messiaen's music is written in a series of what one might call panels. His technique - used many times thereafter - of employing subtly shifting chords, or revisited cycles of notes not unlike a Schoenberg note-row (though more palatable), can make one feel as if the same sequence is being twisted and manhandled over and over again: early on, it assumes the form of a rich, slightly obsessive chorale.

From its beginning, rising from Wagnerian depths, the piece has urgency and a curious forward momentum; at other times (especially the third), it sings, echoing Messiaen's precise evocations of birdsong (Catalogue des Oiseaux, etc.). Initially, it sounds rather like complicated, thickened Poulenc. The orchestra's long-prolonged final notes were especially atmospheric, as were some striking silences, each carefully marshalled by Chailly.

How far Messiaen succeeds in matching, in any sense precisely, the "lofty" New Testament texts that he is reflecting, above all the miracle of the resurrection, featuring an Easter introit and alleluia, is a moot point; and yet there is a sublime feeling, a rapture, about the whole work which is patently uplifting. The woodwind-playing, eager and intricate and yet strangely serene and mollifying, could not have been more rhythmically invigorating. The whole Leipzig team brought with it gleam, precision, and polish: this work positively shimmered.

Chailly's formerly East German ensemble showed the same acuity in Mahler's Sixth, a work of terrifying impact, full of Angst, and yet - not least in the Andante (an Intermezzo, here placed second), which rides like a personal cantilena over angular Mahlerian harmonies - also surely redemptive. Even the pounding of the composer's "blows of fate" - a kind of echoey wooden equivalent of a massive anvil - could not entirely re-establish the glooms.

Much was due to Chailly's penetrating insight into a work that he has recorded to acclaim. A grim pessimism strove to gain the upper hand - and, happily, failed.