Gabriela Montero, LSO St Luke's, London <!-- none onestar twostar threestar fourstar fivestar -->

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It's not the usual classical crowd at LSO St Luke's, and the atmosphere suggests a party, but since we've gathered to witness the ultimate party trick, that's appropriate. On walks a young Venezuelan who announces that improvisation, once a key weapon in the pianist's armoury, is now almost dead, but that she has dedicated her life to reviving it: this evening she'll invite us into her musical world, and will first show what she can do with Bach.

From a simple fugal germ, she embarks on a musical exploration in languid Debussian mode, which becomes Chopinesque; its form is like Liszt's at its loosest, but it has a structure. From another fugal germ she takes off in a different direction, via Rachmaninov to Prokofiev, before ending with a Hungarian dance. It's beautifully played, but these pieces have been prepared, so we're not astonished yet. "Now give me an idea to work on," she says. "Give me a feeling, a state of mind." "Purification" shouts one woman; "Sadness" another. "Ah yes," murmurs Gabriela Montero, "there's so much sadness in the world." She launches into a Beethovenian arioso borne on sighing harmonies, which modulates into Bach. It's beautiful, but not astonishing, since this too could have been prepared.

"Now give me a theme!" At last a proper test? No: the aria from Bach's Goldberg Variations is a soft ball she's been bowled before, but she'll try to do something new with it. The resulting extravaganza grows like a tree, with the melody riding over shifting harmonies, before morphing into a Joplinesque rag: now we're impressed.

She's challenged with a Don Giovanni aria which she transmutes into another Hungarian dance: this is creation on the hoof. "Yesterday" becomes a thicket of immaculate counterpoint; "Summertime" gets a Debussian makeover; "Autumn Leaves" becomes a Palm Court waltz. Then it's dazzle-time: taking the melody and rhythm of a fast Venezuelan dance, she electrifies us as Buena Vista's Ruben Gonzalez used to do.

"I've lost track of time," she says after 80 minutes, before stunning us with her response to a call for "Besame Mucho". Kiss me a lot: yes, a very pleasant world.