I had seen the Simón Bolívar Orchestra before, but never playing Mahler. My first surprise was that they got past the opening page. The Third Symphony begins with a sour blast of ironic brass, fiendishly difficult to bring off without sounding banal or bucolic. The Fifth opens with a trumpet call, faintly mocking the famous triplet-and-minim of Beethoven's Fifth and totally exposed. The best players in Vienna have been known to crack with stage fright. The Venezuelans smiled and breezed it.

None of the players was over 28 or had ever worn concert dress before. Most had been plucked as children from barrios, saved from a short, brutish life of drugs and guns by the redemptive force of Antonio Abreu's sistema, a scheme that offers after-school music lessons as a route to self-worth. 

Its effectiveness has been statistically validated, in social terms at least. One in four Venezuelan students drops out of school before reaching 16. In el sistema, the rate falls to 6.9 per cent. Gangs, who haunt the school gates, respect a sistema uniform. A recent study by Tricia Tunstall, Changing Lives: Gustavo Dudamel, El Sistema and the Transformative Power of Music (Norton, £17.99), estimates that 370,000 children participate in Abreu's nucleos, playing classical music for several hours every day.

Dudamel studied with Abreu from the age of 12, conducting Mahler at 16. I observed him for a week and noted a significant distinction. Where most conductors give expressive gesture and leadership to an orchestra, Dudamel requires live feedback. He gives and, at the same time, receives in return looks of wonderment and encouragement. With the Venezuelans, whom he has known all his life, the result is an ever-rising layering of challenge as each side inspires the other to believe and do better.

He conducted the Third Symphony with a heavy head cold and had to retreat after the first movement to decongest. The tension was unbroken by his absence, so secure was the structure. The Fifth Symphony, two nights later was vividly innovative in tempo relations, allowing the waltzes a dash of salsa and the marches a threat of junta violence. These musicians felt what they played.