Nigel Farage has accused people raising questions about his financial backing of “demonising” him as part of a “coordinated pile-on” to stop Reform UK.

In one of his first speeches since the opening of two parliamentary standards inquiries into his financial support, the Reform UK leader said he had been “dehumanised in the most extraordinary way” in recent months, after the Guardian revealed in April that he had received a £5m gift from the crypto billionaire Christopher Harborne before the last election.

Addressing the audience at the rightwing Conservative Political Action Conference GB (Cpac GB) in London’s docklands, Farage criticised Andy Burnham, the Labour party leader, and the Conservative leader, Kemi Badenoch – labelling them leaders of the “uniparty”.

The event had sessions featuring hard-right figures from around the world. Keynote speakers included the US rightwing influencer Jack Posobiec, who previously promoted the fabricated Pizzagate conspiracy theory smearing prominent Democrats as paedophiles.

To a rapturous audience, Farage said: “Americans will remember this playbook, won’t they? This happened in America. […] I found myself and my colleagues being judged to be guilty, guilty every single day.”

On the day that Burnham officially became the leader of the Labour party, Farage labelled the Makerfield MP a “dud” and said he should call a general election.

“The British public have had enough of this game of musical chairs that is taking place in Downing Street,” he said. “There must be an immediate general election so the country can decide.”

Farage said the election of Colin Sutton, who led several high profile murder investigations during his 30-year-career, as the new Police and Crime Commissioner (PCC) for Norfolk on Friday, was evidence that Reform was the dominant force in centre-right politics. Sutton won with a majority of 14,299, on a voter turnout of 17%.

“We’ve managed to raise more money last year than any other political party, and we are genuinely doing everything we can to get ready to fight that next general election, whenever it begins, and to win it,” he said.

Farage was speaking as the nominations closed for candidates to stand in the Clacton byelection triggered by his decision to quit as an MP, over intense scrutiny over his finances.

“I’ve taken a punt, because I like a gamble,” he said. “The people of Clacton can decide whether they back me or whether they back the establishment.”

Farage’s highest-profile opponent in the Clacton contest is Count Binface, after the competition was boycotted by the other main Westminster parties, who called it a “circus” and a “fake byelection”. Other candidates include the rightwing political activist Laurence Fox and some local residents.

Reform has issued a plea to activists to support Farage, describing his face-off with Binface as a “defining moment for our movement”. A message sent to party figures across England urges them to descend on the Essex town whether they are “just down the road or at the other end of the country”. It said the byelection was of “enormous importance”.

One Reform figure in north-west England, who received the WhatsApp, said it felt like the party had given up on the Greater Manchester mayoralty election to replace Burnham. A poll this week put Reform in third place behind the Greens.

The Reform leader has been the subject of a standards inquiry after he did not make public the £5m gift. He is now subject to another inquiry over allegations that he was financially supported by the crypto gambler and convicted fraudster George Cottrell, who is a close friend.

It emerged on Friday that police were also investigating a donation to Reform’s Robert Jenrick while he was running to be the leader of the Conservative party. Detectives have been handed allegations that five separate warnings were made that a £37,500 donation originated from a foreign donor, the i reported. Jenrick has repeatedly denied breaking any electoral laws.