As an entrance it was unbeatable. The Finnish conductor Leif Segerstam is impressive enough just to look at - nearly as wide as he is tall, with a magnificent mane of hair, voluminous beard and eyes that twinkle halogenically. On reaching the podium he opened the score of his Symphony No 35 wide to the auditorium and flicked through the first few pages. Of course, none of us could see a note from that distance. But that wasn't the point. The score, Segerstam explained, was open to us in another sense. This symphony required no conductor. It was chamber music on a massive scale. We were to relate directly to the City of Birmingham Symphony Orchestra as a creative collective of musicians, while he went off to the side of the stage to play the second piano part.

There was something hallucinatory about the experience. The symphony - subtitled Flashbacking Backwardsly - is mostly a dense collage of sounds. Surging string lines, blazing brass chords, shrill woodwind birdsong and a host of other noises struggle for prominence then fall back into the general melee.

Some of it was baffling, but it was rarely long before some new, striking detail caught the ear. It was hard to believe the orchestra was directorless. Everything seemed to happen on cue; whole sections played as one; intensity and concentration never flagged.

As for that title, Flashbacking Backwardsly, Segerstam's note told us, was written at the turn of the millennium and looked back at recent works (symphonies 26-34, all composed in 1999) as though "experiencing an ending backwards". However confusing the experience may have been from time to time, there was no denying that it was alive, not a bizarrely anachronistic Dada-ist experiment.

There was life in abundance too in Segerstam's performance of Mahler's Ninth Symphony - Segerstam very much the conductor this time. In some ways it was a typical composer's interpretation, bringing out details one might not have noticed before, reconsidering tempo-relations.

The sense of shape Segerstam brought to the long, structurally problematic work was remarkable - as though Mahler had taken lessons in organic logic at work from Segerstam's countryman Sibelius. It was also very moving, in a strangely unsentimental way.

Segerstam may be spectacularly eccentric, but he's a lot more as well. This evening there were more than glimpses of something close to genius.