For an all-too-brief while in the early part of the 20th century, between the fracturing of the Habsburg yoke and the arrival of Nazi jackboots, a native of Moravia – in the new country of Czechoslovakia – could espouse dreams of being part of a wider Slav nation. Leoš Janáček's Glagolitic Mass is a vivid expression of those dreams, written deliberately in a unique style that sought to distance itself from previous works, and sung in Old Slavonic, a language of the early Slav church. ("Glagolitic" is the name of an early Slavic script, invented, according to legend at least, by St Cyril prior to the Cyrillic alphabet still used today).

Janáček was an atheist, and his Glagolitic Mass is blatantly not the work of a religious man steeped in liturgical ritual. Rather, in common with Verdi's Requiem, it is the work of an opera composer at the height of his powers who has been inspired to write a Mass and has thrown every ounce of his dramatic craft into doing so. The result is a work that is massive, gripping, almost overwhelming, and was done full justice last night by Jacob Hrůša conducting huge forces – four vocal soloists, a large Philharmonia Orchestra, Philharmonia Voices, two other choirs and the newly refurbished Royal Festival Hall organ, played by Thomas Trotter.

The work grabs you from the outset, with a huge, brass-laden orchestral fanfare. The choir's entry in the ensuing Kyrie was sensational – singers distributed across the full width of the Royal Festival Hall's ample stage surrounded you with the sound seeming to come at you from every angle. Braying tuba and trombones meld with dark string tones and give way to a plaintive cor anglais solo; pizzicato violins lead in turn to lush chords which, in the succeeding Gloria, become ever more rhapsodic.

The solo vocal writing is unashamedly operatic, with a generous dose of inspiration from Janáček's infatuation with Kamila Stösslová. Most of the work falls to the soprano and tenor: Gun-Brit Barkmin was wonderfully dramatic and had the voice to cut through the massive orchestral sound; Peter Berger was generous, open hearted and mostly powerful enough, albeit unable to make himself heard when the Philharmonia's brass section was in full cry behind him.

The Glagolitic Mass has immense variety through its 45 minutes, with far too many moments of individually brilliant orchestration to list – just about every instrument in the orchestra gets its chance to make a telling contribution. One passage deserves special mention: after the opening fanfare and the five vocal movements, there is an extraordinary perpetuum mobile for solo organ, with insistent phrases that build and build and leave you quite breathless: when it ends, the joyous closing fanfare is almost relaxing.

But this was a concert of two halves, and the first half utterly failed to convince me. Indeed, the quality of the Philharmonia's playing in the Janáček was so far superior to their first half performance that I had to pinch myself to realise that this was indeed the same conductor and orchestra.

Suk's Scherzo fantastique will never have the same potential for high impact as the Glagolitic Mass, but it's still a work with plenty of contrasts and opportunities to thrill. But somehow, these opportunities fell by the wayside. There were some good moments, most notably in the lilting, flowing passages – there's a lovely waltz early in the piece and a sweet-toned dreamy section for flute and harp, to name but two. But outside those lilting passages, I felt no sense of forward momentum – accenting was weak, string sound thin and there were too many instances of individual instruments not quite coming in on the nail.

Dvořák's Piano Concerto in G minor, Op. 33 fared little better. Again, there were good moments, especially from soloist Lukáš Vondráček, who played evenly and accurately; his playing shone in the slower passages in the second movement and in the helter-skelter fugato opening of the third. And there were plenty of showings of Dvořák's ability to breathe life into the orchestral wash with small woodwind fills. But overall, the concerto failed to grip. It's a work that's nearly as long as the Glagolitic Mass, but which, in this performance at least, transmitted a small fraction of its energy and exceitement.

This was one of those concerts where the idea of a single star rating collapses. Had I left at the interval, I would have done so with memories of a distinctly lacklustre evening. Had I arrived at the interval, I would be telling a story of unalloyed excellence. As it is, I will be choosing to remember a truly thrilling rendition of a choral work that is giant in scale and unique in style, which I would happily recommend to all comers.