A smattering of confused applause greeted gay rights activist Peter Tatchell to the Barbican Hall’s stage before the first instalment of the London Symphony Orchestra’s Berlioz cycle under Valery Gergiev. Stating that he did not wish to disrupt the performance, Tatchell made it clear he was there to protest Gergiev’s noted unwillingness to criticise Vladimir Putin’s record on human rights, particularly with regard to Russia recently prohibiting the dissemination of “propaganda of nontraditional sexual relations to minors”. Such demonstrations are nothing new to Valery Gergiev, whose concerts over the last few months have frequently been the subject of such protests, often far more disruptive than Tatchell’s restrained effort. The security (and an irate principal trumpet) having removed Tatchell from the platform, an apparently untroubled Gergiev came to the platform and launched a seemingly possessed LSO into some of the most thrilling Berlioz one could ever hope to hear.

Juxtaposing two youthful works, the Waverley Overture (1827–8) and the Symphonie fantastique (1830), with the more mature, considered Les nuits d’été (1841, orch. 1856), it was clear from the get-go that this was a performance totally in tune with the fervent, prescient imagination of this bona-fide Romantic polymath. The Waverley Overture is based on Walter Scott’s novel of the same name; two of the young, dreaming knight’s lines head the full score – “Dreams of love and lady’s charms / Give place to honour and to arms”. From a lyrical, nocturnal cello solo in the opening to furious gallantry and naïve enthusiasm in the Allegro, the main body of the work, this is clearly the music of a young man who shares with Waverley a penchant for reverie. In Berlioz’s overture, though, high seriousness gives way to ribald good humour and infectious joie de vivre.

Gergiev and the LSO had plenty of each, with a quick tempo even in the slow introduction never robbing the orchestral sighs of expressivity, and an exhilarating Allegro, hair-raisingly fast but always graceful and Mendelssohnian, was life-affirming in its unselfconscious vivacity. Never did the energy sag even for a moment, the LSO’s famous rapport both with Gergiev and with Berlioz – thanks to his greatest champion, the late Sir Colin Davis – was firmly on display. Some bizarre dynamics, early intimations of the musical grotesquerie (firmly entrenched, of course, in Berlioz’s admiration for Beethoven) were embraced with aplomb, and the flabbergasting technical security of the orchestra was never allowed to replace total musical commitment.

What’s more, never was this mere sensationalism. Alongside mezzo-soprano Karen Cargill, the LSO and Gergiev gave an account of Les nuits d’été demonstrating their deep understanding of Berlioz’s troubled Romanticism. A setting of poetry by the Romantic-cum-Symbolist Théophile Gautier, the cycle explores different aspects of romantic love, from the moving simplicity of being together in nature to the agony of mourning, and rounds it off with an ironic jab at all that has come before in “L’île inconnue”, set by Berlioz in largely outward fashion. Cargill’s voice was infinitely flexible, finding for the most part a tone with just enough voix naturelle as to remind one of the cycle’s intimate beginnings as a voice-and-piano work. However, a full-bodied operatic final song and excellent dramatic presence gave her performance as richly varied a range as Berlioz’s often proto-“Impressionist” score demands.

So to the Symphonie fantastique, a work played so much as to render one almost tired of seeing it on concert programmes. Under Gergiev’s baton, though, this warhorse of the repertory seemed completely inspired, leaping from the page before the players’ eyes; as if suspended in the same effluvium of opiates from which Berlioz’s ideas proceeded, lovers’ sighs, corybantic revels, and ghastly sonic excrescences buffeted the audience’s ears. No apologies for utter musical indulgence from Gergiev; this is the composer, after all, who suggests an appropriate number of harps for a grand, national festival orchestra would be around 30, if maybe not for the Symphonie fantastique. Perhaps disappointingly, only two were used, though staggeringly full tone from both in the second movement – well displayed in Gergiev’s slower, truly waltzing tempo – meant they were quite sufficient.

Tempos both madcap and relaxed were unutterably, gloriously louche, with a highlight being Fabian Thouand’s oboe solo in the first movement. After the exhausting peaks and troughs, the relentless energy of what came before, a significantly slower tempo from Gergiev and irresistible tone from Thouand made this moment truly valedictory, a turn to the spiritual, reaching a transcendent apotheosis in the “Religioso” close of this long movement. With total belief in the score, Gergiev gave total freedom to and Berlioz’s visionary sonic effects, ideally realised through the overwhelming brilliance of the LSO. With utterly sincere lyricism from cor anglais Christine Pendrill alongside ear-splitting, monstrous brass fanfares, this was a performance which, whilst totally committed to Berlioz’s uncanny ideas, was just as original and brilliant as the man himself.