The Birdcage pub in Stoke Newington, north London, seems an unlikely birthplace for a rebellion. On a midweek afternoon, the bar is almost empty. Spring sunshine streams through the windows; Spandau Ballet provide a gentle soundtrack; black-hatted men from the local ultra-Orthodox Jewish community pass by outside.

The only customers are four men sitting at a table in an alcove. It was at this spot, more than five months ago, that this group of friends came up with an idea born from their collective despair over the “lies, lunacy and hypocrisy” of the Brexit process.

That idea was to emblazon the 2015 tweet by the then prime minister David Cameron, “Britain faces a simple and inescapable choice – stability and strong government with me, or chaos with Ed Miliband”, across a giant billboard. Since then, it has turned into a national campaign that has captured the imagination of millions of people, sparked heated debate and infuriated Nigel Farage and his supporters.

Now, the creators of Led By Donkeys have decided to go public. Ben Stewart, James Sadri, Oliver Knowles and Will Rose are hitting back against claims by rightwing bloggers and trolls that they are a shadowy, anonymous group by revealing their identities and motives. “It feels like the right time to say who we are and defend our work and views. We’re proud of what we’ve done,” said Knowles, 43.

The four have been targeted by the Guido Fawkes website, which claimed to have “unmasked” them as “Greenpeace campaigners” last week after they registered their names with the Electoral Commission. Stewart and Knowles do indeed work for Greenpeace, and Sadri and Rose have previously worked for the environmental campaign group, but Led By Donkeys is an entirely freelance amateur operation. “No one at work knew we were doing this,” said Stewart, 45.

A poster that went up in Leeds in January. Photograph: Steve Morgan

The men met for a pint at the Birdcage in mid-December, just after Theresa May had postponed the first scheduled Commons vote on her Brexit deal. “It felt like the country had reached another level of chaos,” said Stewart. “We were looking at Cameron’s tweet from before the 2015 election, which was doing the rounds again, and pissing ourselves with laughter. We said it would be a real shame if he deleted it because it encapsulates the total failure of Britain’s political leadership.”

Their eyes alighted on an advertising billboard across the road. Within a few days, they had ordered five billboard posters, each in 12 sections: tweets and quotes from Cameron, Michael Gove, Liam Fox, John Redwood and David Davis. And they chose the campaign’s name from the phrase “lions led by donkeys” used to describe first world war soldiers and their incompetent generals.

In January, the first poster went up on “borrowed space” in Stoke Newington, covering an ad for the Halifax. The group had invested in hi-vis jackets (in the hope of looking legitimate), a ladder, buckets and wallpaper paste, but they had no idea how to actually get the printed sheets up on the giant board.

“The sheets kept falling down, drivers were stopping to look at us, we were covered in paste, and it took us an hour and a half. But then we stood back and looked at it. Olly put his arm round my shoulder and said: ‘This is absolutely fucking beautiful’. We took a picture to post on social media, expecting maybe seven retweets. And it just exploded,” said Stewart.

Sadri, 39, said: “It was a way of channelling frustration into action and holding some of these politicians to account with a bit of humour. There’s a lot to be said for laughing at those in power.”

The group has also been following Farage’s March to Leave from Sunderland to London with two digital ad vans displaying a selection of tweets and declarations.

By 4pm the next day, the Led By Donkeys poster was gone, pasted over by plain blue paper. But its life on social media was just beginning.

Their next venture was in Dover, a town on the frontline of Brexit and a Farage stronghold. In deserted streets and feeling “pretty nervous”, the group put up four posters in one night, again tweeting photographs of their handiwork. The posters were stripped off the next day, “but we were getting thousands of Twitter followers – and loads of people were saying ‘we’d love to throw some money at this’,” said Knowles.

They set up a crowdfunding appeal with a target of £10,000. It was reached within three hours. “We increased it to £50,000, and we’d hit that by the next morning,” said Stewart. By the middle of last week, they had raised £438,704 from 14,299 supporters – an average donation of £30, many of them much smaller amounts and a handful of £1,000 gifts.

With money in the bank, the group decided to go legitimate. The cost of a billboard ranges from £500 to more than £1,000, depending on location, size, how long the ad is displayed for, and lighting. “But the big [billboard] firms said they didn’t want anything to do with us. Luckily we found some independents willing to take us – and now some of the big companies have come round,” said Sadri.

Stewart added: “We started dishing out a lot of money, and tweeting everything we did, and the whole thing got bigger and bigger. And we were doing all this in the gaps in between our day jobs and our kids. I’d be on our WhatsApp group at 6am, then suddenly the baby monitor would go off. It’s been pretty exhausting.”

The banner bearing a David Davis quote was carried by thousands of marchers at the Put It To The People rally in Parliament Square. Photograph: Jiri Rezac/Led By Donkeys

A big moment came with the People’s Vote demonstration in March. The group had the idea of a huge lightweight banner to be passed over the heads of marchers, taking their inspiration from football supporters. A 40x20 metre banner, weighing 160kg, was made in just five days at a cost of £12,000. Hiring a helicopter for an hour to photograph it cost another £5,000, and the exercise needed a team of 25 volunteers to help marchers pass it aloft.

The quote they chose was from Davis, the arch-Brexiter: “If a democracy cannot change its mind, it ceases to be a democracy.”

The comedian Steve Coogan introduced the piece of political theatre to the crowd. “They responded immediately. There were big smiles. Everyone had their hands in the air to help with the banner. It was a beautiful moment of coordination and cooperation – the antithesis of Brexit,” said Stewart.

“This was 10 weeks after we’d put up our first poster, and I remember looking up and thinking, that helicopter is the BBC and that other helicopter is Led By Donkeys. How the hell has that happened since we went down the Birdcage and had a pint?”

Led By Donkeys had also been following Farage’s March to Leave from Sunderland to London with two digital ad vans displaying a selection of tweets and declarations. “Farage couldn’t avoid them. It was a pretty special moment, directing the vans into shot when he was speaking,” said Rose, 40.

The posters were a way of channelling frustration and holding some of these politicians to account with a bit of humour. James Sadri

Farage and his supporters have struck back, accusing Led By Donkeys of breaking Electoral Commission spending rules. Unregistered non-party groups may only spend up to £20,000 in England during an election campaign, while registered campaigners are permitted to spend almost £160,000.

In the official campaign period, Led By Donkeys has spent £18,298 on its European election push, in accounts seen by the Observer. But Stewart, Sadri, Knowles and Rose decided to register with the commission in order to be “proactively transparent”. They have also disclosed their previous activism: in 2008, Stewart and Rose were among six people cleared of causing criminal damage at a coal-fired power station in Kent, and Knowles has a number of convictions for peaceful protests.

In the past couple of weeks, they have also come under fire from the left over a billboard showcasing a quote from Ann Widdecombe, the former Tory minister who has joined the Brexit party: “Homosexual acts are wrongful.” Most people, their critics argued, would simply see the homophobic message and miss the subtleties of Led By Donkeys’ messaging.

One of the first posters to go up, in January. Photograph: Led By Donkeys

The group swiftly apologised and had the posters taken down. “It was a mistake. We looked at the reaction on social media and it didn’t take long to realise the criticism was justified,” said Stewart.

Some have also suggested that Led By Donkeys operates in an echo chamber, only striking a chord with Remainers. “People have said we’re inside a bubble, but we’ve deliberately chosen Leave-voting areas to put up our posters. We know we’ve sparked hundreds, thousands of conversations. Loads of people have messaged us to say the billboards have kicked off discussions with their dad or their uncle or someone else. You have the whole mix of society walking past a billboard – you can’t have a billboard in a bubble,” said Stewart.

Asked what comes next, the four are uncharacteristically silent for a moment. “The polarisation and division in the country really bothers me, and we talked a lot about whether our poster campaign exacerbates that. We were trying to create a conversation between Leavers and Remainers – and the evidence from what people have told us is that we’ve done that,” said Stewart.

“But it feels like the country is about to slip into an even more polarised state, with a revoke or no-deal dynamic emerging. What can we do about that? I don’t know. But you have to hold people accountable.”