The lows have it: the Mariinsky's Boris Godunov at the Barbican

Russians sing bass like no-one else. The more so when it’s in their own language – indeed, singing one of the great works in that language, Mussorgsky’s Boris Godunov. And more still when we have the Mariinsky's top singers on top form.

It wasn't just the soloists. The Mariinsky Chorus, augmented by the Tiffin Boys' Choir, may not have been present in huge numbers, but the sound they produced was sensational. One generally worries about an opera chorus being drowned out in a concert performance – after all, they're behind the orchestra, without the protection of an orchestra pit. Not here. Even when Valery Gergiev had the orchestra turned up to full throttle, with searing strings and blazing brass, the chorus simply blew them away.

Boris Godunov is a unique piece in many ways. Basically, it's an historical epic, but the episodes that are presented and the angles from which they are viewed are idiosyncratic and diverse. The grand crowd scenes are anything but pure pageantry, as the police officer whips the bickering masses into yelling what they're told: but when the crowd starts singing for real, the harmonies seem to come from the very soul of Russia. The palace scene of Boris and his family achieves parent-child intimacy of which Verdi would have been proud, all the more pathos-filled because we know the tragedy that is to come. The scene at the inn where the pretender Grigory escapes from the illiterate policeman combines historical import with high comedy and a particularly Russian irreverence for petty authority. Where operatic convention gives mad scenes to women, in this work, it is the Tsar Boris himself whose mind disintegrates. And the scene that really put shivers down my spine was the one in which the Holy Fool treats the Tsar as a murderer to his face, Boris pardons him enjoining him merely to "pray for me", and the Holy Fool refuses, explaining that the Virgin Mary will not permit him to do so.

That's a scene present only in the version performed here, namely Mussorgsky's 1869 original, which was refused by the censors. It's shorter and tauter than the 1872 version with which Mussorgsky eventually won acceptance, and today, it seems more strikingly radical. Three quarters of a century before Britten made it his norm, the score has an exceptional multiplicity of timbres, each one matched to the action, just as the phrasing of the music is individually tailored to the speech patterns of each character. And the harmonies, which include snatches of pastiche of ancient Russian liturgy, sound unlike anything from Western Europe.

There were phenomenal performances from both Mikhail Kazakov, in the title role, and Mikhail Petrenko as his unlikely nemesis, the monk Pimen. Kazakov fully explored every aspect of the despot's complex character, particularly compelling in showing Boris' break-up. Petrenko sang Pimen quite beautifully, with gravelly depth and glorious lyricism. The young bass Yury Vlasov was both musically vivid and dramatically exciting as the vicious Nikitich. As the vagrant Varlaam, Sergey Alexashkin is getting too old to give of his best musically, but hammed up the part with gusto – which worked OK for me, given that the character of a drunkard masquerading as a monk is such an obvious object of derision. Few operas have such richness of bass voices and I could not ask for a better quartet.

Amongst the other voices – another unusual feature of Boris is that most scenes have their individual set of stars who play little if any part in the rest of the opera – Anastasia Kalagina impressed as Boris's daughter Xenia: clear-voiced and sweet in her high register.

If I have one cavil about the evening, it's that it sat slightly uncomfortably in between a concert and a semi-staged performance. Basically, it was a true concert performance, with evening dress and music stands. But for that to work properly, we needed a libretto to follow: the surtitles provided gave no indication of which character was singing, so anyone who hadn't read the libretto in advance would have been mystified, particularly by Ekaterina Sergeyeva singing the trouser role of Prince Fyodor in a sweet, feminine voice and a full satin ball gown. Many of the singers chose to act their roles as well as sing them, most notably Kazakov, whose marvellous portrayal of Boris could not have been achieved without doing so. But not all: others entered the stage, walked to their music stands and sang.

On the grand scale, though, that's a minor detriment to a truly extraordinary evening of music. When Mussorgsky and Gergiev cry havoc and let slip the dogs of war, they truly knock you back into your seat. This may not have been the most note-perfect of performance, but you couldn't fault it for raw emotional intensity. This was a great showcase for one of the masterworks of Russian music.