The third hammer blow of fate has been struck by an enormous mallet on a wooden crate high in the orchestra. The music withers slowly away: after a final crashed chord and timpani roll, the conductor signals total hush from both orchestra and audience for a few moments before the ritual applause can begin. Thus ends the 90 minute behemoth that is Mahler's sixth symphony, an emotional, draining experience for a huge Albert Hall audience and a huge BBC Symphony Orchestra - nine horns, seven percussionists, four harps - the list goes on.

Mahler's sixth is both easier and harder to grasp than many of his other symphonies. Easier, because the classical structure is reasonably clear: themes are stated, developed, interleaved with contrasting themes and recapitulated. There's less of the requirement to grapple with polyrhythms: the harmonies may be complex and subtle, but mostly, there's only one main thing to listen to at any given moment. Harder because of the symphony's sheer scale: you are required to fully concentrate on an hour and a half stretch of powerful, dark, brooding music, keeping aural track of its panoply of shifting motifs. It's a demanding task even for a dedicated lover of orchestral music.

I'm not sure why the Proms felt the need to add anything else into the programme, let alone Richard Strauss's early piano concerto the Burleske in D minor, which is hardly one of his more distinguished works. The showy piano part was handled well enough by Kirill Gerstein, and the quirky orchestral parts successfully negotiated by the orchestra. But I suspect that everyone concerned (except perhaps the Royal Albert Hall bar concessionaires) would have preferred to move straight on to the main event.

Much of Mahler's music shows a fondness for the military bands of his childhood, and the sixth symphony starts with a massive, awe-inspiring military march. But this is no cosy town square parade: this is the sound of soldiers marching into battle, tramping across a desolate, wasted countryside. The symphony was written in 1903-4, a decade before the devastation of the first world war, prompting some commentators to muse upon Mahler's prescience. The military themes alternate with sublime contrasting sections, including one of the tenderest of Mahler's themes, written as he thought of his beloved wife Alma.

Conductor Semyon Bychkov is a bundle of energy and did an admirable job of keeping the orchestra on the pace and solidly together. The tempo never flagged, and the music drove relentlessly on with the barest of pauses for the calmer interludes. It was an impressive performance, albeit one that fell short of some of the very highest standards that the Proms has seen this year. The string section was powerful and well phrased but lacked that last edge in sound quality, and the brass and woodwind players didn't always display the highest level of individual instrumental virtuosity. The wind parts in Mahler are filled with little quotes and interjections: some of the solos came off beautifully, but not all.

The scherzo (played as the second movement here - it's often swapped around with the andante) evokes the military themes of the first movement, distorted in harmony and rhythm and alternating with a calming trio section. The andante is peaceful and lovely, but as always with Mahler, suspended chords and departures from harmonic convention keep the listener slightly off balance and restless.

And so to the gigantic finale: at 30 minutes, itself as long as the complete symphonies of earlier years. It's a complex recapitulation of many motifs of the earlier movements, with some new material added. Mahler adds a number of signposts to show the listener the way: some swooshes of the four harps mark section boundaries, and the famous three hammer blows denote the power of fate. Again on the subject of prescience, much was made by Alma of the three horrors that befell Mahler the year after the première, conveniently omitting from the list his devastating discovery of her affair with Walter Gropius (as Stephen Johnson points out in the programme notes). But the fourth movement is not a work for beginners: there is such a huge amount of sound and there are so many waves of music hitting you with such violence, rolling away into quiet passages only to redouble with extra force. If you don't know the music well, it's hard to maintain concentration throughout, and a few audience members, including the couple in front of me, couldn't cope and left early.

For those with the staying power, it was a magnificent ending: utter calm and stillness at the culmination of a massive, draining emotional roller-coaster. Everyone, orchestra and audience alike, looked drained - with the possible exception of Bychkov, who seemed to take it all in his stride, a bundle of energy to the last.