The star conductor of the symphony of 385,000

Two orchestras, two countries and nine-and-a-half symphonies: Gustavo Dudamel's Mahler Project is a typically epic affair. Clemency Burton-Hill meets the maestro

It's a balmy night in downtown Caracas and, backstage at the Teatro Teresa Carreno, a hexagonal concrete confection that is South America's second-largest concert hall, the conductor Gustavo Dudamel is grabbing a last-minute look at an orchestral score. "Which one is it tonight?" he quips, with his trademark dimpled grin. "I am losing count..."

As well he might. The world's hottest maestro is midway through the "Mahler Project", a double-symphony-cycle – about to be screened in British cinemas – which brings together both sides of his "musical family", as he puts it: the Los Angeles Philharmonic, where Dudamel, 31, has been the music director since 2009, and the Simon Bolivar Symphony (formerly Youth) Orchestra, where he has been the music director since 1999 and which comprises his friends and fellow graduates from El Sistema, Venezuela's remarkable national music education programme.

The Mahler Project is an undertaking of breathtaking ambition: two orchestras; two cities; two countries; five weeks; nine-and-a-half symphonies, each performed twice; dozens of outreach projects; 200 musicians; 2,000 singers; one conductor, working non-chronologically, from memory. No wonder his head is spinning.

"It is a little crazy," Dudamel concedes later, after the concert. Outside on the plaza, next to the giant screens erected for the overspill crowd, hawkers are still doing a brisk trade in Gustavo T-shirts, Gustavo chocolate, Gustavo badges, Gustavo CDs, and Gustavo wigs. Outside his dressing room, meanwhile, two hefty bodyguards hold the many well-wishers at bay. Surrounded by his grandmother, his mother and his wife Eloisa – who has recently given birth to their first child, Martin – Dudamel nods and those famous black curls bounce in apparent concurrence. "Crazy, but amazing..."

This is certainly an apt description of the Mahler Experience, of which British audiences can now get a spectacular taste. From today, cinemas across the UK will be showing a film of the project's triumphant finale, at the centre of which is a performance, recorded live in Caracas, of the Eighth Symphony so gargantuan that its nickname, Symphony of a Thousand, seems, for once, conservative. Made up of El Sistema students from nucleos (local community music schools) across Venezuela, the chorus alone numbers well over 1,000 – "maybe 1,200, maybe 1,300?" shrugs the choirmistress, Lourdes Sanchez, when I ask, as if this is an entirely reasonable number. It's hardly surprising that nobody can quite keep count: this is a country, after all, in which more than 385,000 children are engaged in free, comprehensive El Sistema music education programmes; 81 per cent of them hailing from the country's lowest-strata barrios.

Ever since Dudamel and the Simon Bolivar Youth Orchestra stormed the 2007 Proms – a YouTube video of which promptly went viral – British audiences, like many all over the world, have been dazzled – not only by the contagious joy and kinetic energy of El Sistema's performers, but by its compelling message, which has at its core a radical vision more concerned with social transformation than with creating musicians of the stature of Dudamel. Sir Simon Rattle once called El Sistema "the most important thing for the future of classical music", an "emotional force of such power that it may take some time to assimilate what we're seeing and hearing."

Dudamel and "the Bolivars", as they are fondly known, will open the London 2012 Festival, in Scotland, on midsummer night, after having led a week-long residency at Big Noise in Raploch, one of the UK's grimmest sink estates where an El Sistema-inspired initiative has been yielding remarkable results since 2008.

By now, we know that Dudamel and his compatriots will put on a spectacular show; we also know that the quantum leap of imagination with which economist-conductor Jose Antonio Abreu founded this extraordinary programme in 1975 – with, famously, 11 kids in a garage – was one of profound, humane vision. With more than two million Venezuelan lives demonstrably transformed through El Sistema, the Inter-American Development Bank, which recently stumped up a $150m loan to expand the programme across the country, calculates a significant return on investment – $1.68 for every dollar invested.

As he accompanies me, along with three bodyguards, across a busy Caracas street in broad daylight to Parque Centrale, where the administrative offices of Fundacion Musical Simon Bolivar (the official name for El Sistema) are held, the foundation's director for institutional development Rodrigo Guerrero reminds me that, according to the bank's report, each child engaged in El Sistema is four times more likely to finish high school than those who aren't. For each child in the programme, moreover, an average of three additional adults in the community are also positively affected. There is a reason why seven successive governments have funded the scheme; and why Abreu, in a rare private interview in Caracas, tells me that he will not be satisfied until every child in Venezuela has the opportunity to enrol at a nucleo.

The Mahler Project, meanwhile, which Dudamel explains is "a dream come true" for him, may be a "monumental" musical extravaganza, but he clearly views it as more than just another concert series. By bringing together both sides of Dudamel's musical life and reimagining the possibilities for cross-cultural musical collaboration on an epic scale, it marks the next chapter in the global El Sistema revolution.

"This is not only a musical event but a great human event," explains LA Philharmonic president Deborah Borda, herself something of a force of nature. "We are trying to define what an orchestra of the 21st century is going to be." Formerly of the New York Philharmonic, and one of America's most formidable orchestra executives, the elegant 62-year-old has been instrumental in realising Dudamel's "dream" by instituting the largest El Sistema-inspired programme in the US, Youth Orchestra Los Angeles, and tirelessly facilitating the expansion of similar initiatives internationally. Over fantastically strong local Caracas coffee, she elaborates for me a wider philosophical mission, of which the Mahler Project is only the start; and which could – and should – have ramifications for any society that cares not only about the future of its cultural life, but about the possibility of widespread social change and the alleviation of poverty, both spiritual and material.

"You can't run big arts institutions in the way you did 15 or 10 or even five years ago," Borda asserts. "It used to be that you could just focus on the artistic imperative, but today there is a social imperative too and we must recognise that and act upon it. I see it as potentially, could this be the Trojan horse for classical music? Could we finally ignite and capture people's hearts and imaginations, move them forward?"

"Gustavo is obviously the apotheosis of El Sistema and a pivotal figure in the music today, but he is also a 21st-century thinker," she insists. "He is not in the model of the old maestro, the depth of his thought is quite something. He understands that this is the age of communication, of collaboration. The age of access: from cyber waves to making spiritual, as well as material, abundance available to people."

The Mahler Experience opens at the Electric Cinema in London today and plays on 16 and 19 April around the UK (