Nigel Farage has defended a new immigration-centred Ukip poster campaign as "a hard-hitting reflection of reality" after it was attacked as "racist" by political opponents.
The anti-European Union (EU) party is using £1.5 million of funding from millionaire ex-Tory donor Paul Sykes to launch its biggest-ever publicity drive ahead of European Parliament elections on May 22.
To be displayed at hundreds of billboard sites across the country, they carry stark warnings that "British workers are hit hard by unlimited foreign labour" and that 26 million unemployed people across Europe are "after" UK jobs.
Under the slogan "take back control of our country", others complain that 75% of British laws are made in Brussels and that UK taxpayers fund the "celebrity lifestyle" of EU bureaucrats.
Critics compared the immigration posters with those used in the past by the far-right British National Party.
Labour MP Mike Gapes said they were "racist" and appealed to "all decent British Commonwealth and EU citizens" to register to vote in May's polls.
But Mr Farage, whose party is tipped to push the Conservatives into third place and perhaps win the election outright, dismissed the concerns of the "chattering classes".
"These posters are a hard-hitting reflection of reality as it is experienced by millions of British people struggling to earn a living outside the Westminster bubble," he said.
"Are we going to ruffle a few feathers among the chattering classes? Yes. Are we bothered about that? Not in the slightest.
"Ukip is hugely grateful to Paul Sykes for his magnificent contribution to the great cause of restoring Britain's ability to be a self-governing nation. The political earthquake I have spoken of is on its way."
Mr Sykes said: "I am supporting the biggest advertising campaign in Ukip's history to bring home to the British people what is at stake.
"The European elections are the most important for many years.
"We have the chance to support a party that represents a complete break with the past. The other parties, whatever their merits, are content to work within the existing Brussels straitjacket.
"They cannot do anything about immigration or British workers being undercut by cheap foreign labour and they are prisoners of the European Court of Justice and the closely-related European Court of Human Rights, which stops us deporting foreign criminals and terrorists.
"They are about to embrace new European controls over our policing and justice systems - including the European arrest warrant, they allow interference in our tax system and they subcontract more and more of our foreign and defence policies to unelected EU bureaucrats.
"An overwhelming victory for Ukip will break the political mould in the UK, forcing Labour and the Lib Dems to back a full-scale referendum and intensifying the popular pressure for that to be staged as early as general election day 2015."
The posters will be displayed in two waves over the next four weeks and be accompanied by adverts in digital media.
Ukip said today that its fast-rising membership had now passed 36,000 - only around 8,000 behind the Liberal Democrats and on course to overtake Nick Clegg's junior coalition party by the time of the 2015 general election.
Mr Clegg appealed for help from Labour and pro-EU Tories to counter the eurosceptic party's arguments in the run-up to May 22 and dismissed Mr Farage's claims to be an insurgent.
He wrote in The Guardian that Ukip is part of the anti-Brussels "establishment" and its leader is the sort of professional politician he accused others of being,
"Of all Nigel Farage's far-fetched claims - and there are many - the most outlandish is the idea that Ukip's call for an exit is the insurgents' battle cry," he said.
"European withdrawal is presented as a great revolutionary promise, held in stark contrast to the status quo upheld by a homogeneous political elite.
"What poppycock. For a start, Farage is every bit the professional politician he enthusiastically reviles. He and I were elected to the European Parliament on the same day in 1999. I left after five years. The Ukip leader is still there.
"More important, there is nothing remotely new about his party's ambitions. Ukip is simply the fresh face of a long-standing Eurosceptic establishment, supported by many in the Tory party and significant parts of the press."
Mr Clegg acknowledged that a British exit from the EU was now "plausible" but insisted he would happily take on Mr Farage in more televised debates - despite being widely seen as having lost support to him after the two already broadcast.
"I would meet the Ukip leader in front of the cameras every day between now and polling day if I could," he said.
"Prior to these debates, the case for in was largely absent from the public debate. Twenty years of deliberate inaccuracies and entrenched mistruths are not going to be reversed in two hours."
Admitting the pro-EU case lacked "volume", he said: " The Lib Dems have started this debate - but we cannot win it alone.
"We want to work with others to deliver the firepower needed to challenge the Eurosceptic establishment.
"If Labour is still a pro-European party, it needs to come off the fence. Tory modernisers must risk the wrath of their backbenchers and speak out."
He added: "If you are holding back, ask yourself: what are you waiting for? The fight is on. The threat is real. It's time to pick a side."