There was a tussle over the last cardamom flapjack at the cake stall, there was some bitterness from those who had paid the full £35 for a ticket while others got in for free – but there was one thing everyone at this nice middle-class festival in north London could agree on: the establishment was absolutely terrified of them.

“The establishment hate the idea we are coming together to talk about politics,” boomed the Unite general secretary, Len McCluskey, speaking at his party’s music and politics festival, Labour Live, on Saturday, to an audience of largely white middle-class men clutching £4 beers. “I congratulate our leadership for having the bravery to put this on”.

'This is our music; these are rebel songs,' a sign said behind them, as their gentle harmonies washed over the field

“Look around you: we are some of the scariest people, we are feared by the establishment,” said a fellow panellist, the Labour MP Dawn Butler, before taking audience questions: a woman who pointed out that hers was the only foreign accent she had heard at the event so far, and a former councillor candidate who wanted specific mental health resources for “people who have stood in elections and failed to win”.

Meanwhile, McCluskey homed in on the source of the fear. “The Tories fear Corbyn,” he said. “If Brown or Blair were leading the party now, we’d have already had an election, because the establishment wouldn’t fear them.” This must be because Labour leaders who have made it to No 10 have historically been no threat to the establishment, no threat at all. After all, what could be more establishment than being prime minister?

Other groups afraid of the festival – which featured a performance of gentrification-themed spoken-word poetry – included a “sea of hostile media”, who were at that very moment raging against its audacity. Unless they were one of the panel speakers, of course, about a quarter of whom were journalists who work for the national media.

Facebook Twitter Pinterest ‘Their leader’s presence was enough: they did not care what he said.’ Photograph: Chris J Ratcliffe/Getty Images

Meanwhile, headliners the Magic Numbers, known to your dad for their mellow love songs, made it clear their performance was one in the eye for the man, too. “This is our music; these are rebel songs,” a huge sign said behind them, as their gentle harmonies washed over a field of families digging out lunchtime Tupperware. “Are you in or out?”

In between events, buoyed by how livid they were making the powers that be, festival-goers defiantly browsed the stalls: stocking up on almond polenta cake, plastic bottles of wine (£20), and gift cards bearing the slogan “first tea then revolution”, while in the background a guitarist on the main stage sang about “kicking posh boys up the arse”.

But the main event - most terrifying of all to the establishment - was yet to come: the appearance of the great and powerful wizard himself. As the crowds gathered rapturously by the main stage, a full 40 minutes before his appearance, it became apparent the whole festival had been something of a warm-up act. First, some stirring songs and chants, then, a short film: Jeremy being applauded by thousands, Jeremy being adored by young female activists, Jeremy walking purposefully along the road to rap music. And finally, an introduction from the shadow chancellor, John McDonnell, explaining how annoyed the Daily Mail would be by all this. By the time the leader appeared, they were going absolutely ballistic.

Labour Live: for the few, not the many Read more

And then he started to speak, and it suddenly became apparent why all day his supporters had been talking him up as a fearful threat to the system. The man behind the curtain was no danger to anyone, but a muddled and feeble speaker – rambling in a strained, breathless fashion, in sentences such as this: “This is the first time a political party has ever organised something like this: bringing together music, poetry, creativity; bringing together the inspiration that is there in all of us, to learn from each other, and be strengthened, by that learning, from each other.”

Deprived of natural pauses, or sentences that felt like they had ended, the adoring crowds took to cheering whenever Corbyn happened to take a breath. Their leader’s presence was enough: they did not care what he said. But one suspects other voters will be more circumspect.

And this is the point. The festival was a show of Corbyn’s power among hardcore Labour supporters – in the event the party claimed some 13,000 of their 20,000 tickets were sold, despite predictions it would be a total failure. It is difficult to imagine another of the country’s political leaders drawing in such a crowd. But politics is about winning over people who do not already agree with you. And as far as this goes, the Tories shouldn’t be too afraid just yet.

• Martha Gill is a freelance political journalist