The Liberal Democrats appeared to be the biggest victims of the "earthquake" in British politics caused by Ukip, with Nick Clegg's party struggling to hold on to any of its MEPs.
Mr Clegg gambled by taking on Nigel Farage in head-to-head debates ahead of the European elections and highlighted his support for the Brussels project by casting the Lib Dems as the "party of in".
But the Lib Dems lost seats across England as the main parties conceded defeat in the face of a Ukip surge and the Deputy Prime Minister will face further questions about his leadership.
Mr Farage claimed victory, telling Sky News: "Ukip is going to win this election and, yes, that will be an earthquake because never before in the history of British politics has a party that will be seen to be an insurgent party ever topped the polls in a national election."
A senior Lib Dem source s aid it was "touch and go" whether the party would be wiped out in the European elections.
"It's not looking good," the source said.
Party president Tim Farron, seen as a possible successor to Mr Clegg, said the Deputy Prime Minister should not face a leadership challenge despite a "staggeringly disappointing" European election result.
The Lib Dems had "paid the price" for the decision to fight a strongly pro-European campaign, he said, but he told Sky News that Mr Clegg should be praised for having the backbone to take an unpopular position over the EU and confront Ukip's "threat to British cultural values".
Mr Clegg has insisted he is staying despite the party losing more than 250 councillors in local elections and being on course for a rout in the European elections.
Southport MP John Pugh suggested that a dozen of his Commons colleagues had expressed doubts over whether Mr Clegg should continue at the head of the party.
Ex-MP Sandra Gidley - one of around 250 people to have signed an online letter demanding a change of leader - said Mr Clegg was tainted by his position in the Tory-led coalition.
In Sheffield, the city where Mr Clegg is an MP, the Lib Dems trailed in fifth in the Euro-elections.
Labour won with 47,571; Ukip 39,139; Greens 17,288; Conservatives 15,329 and Lib Dems 14,299.
The Lib Dems, who went into the contest with 11 MEPs, lost two seats in the Yorkshire and the Humber region, and one each in the East Midlands, Eastern and North East.
At the count in Southampton Guildhall, Mr Farage said: " We have formed the people's army to fight the establishment.
"I love Europe, it's the European Union I have a problem with."
Dismissing the suggestion that tonight's result was a protest vote and Ukip's "high point", Mr Farage said: "In 2009 it was the high water mark and how we would never do it again in a domestic election, then we blew them down last year and now there's another protest, so no, I do not think so."
When asked if he would target Kent to stand in the parliamentary elections next year, he said: "I haven't really thought about it. I have always said it will be somewhere in the South East. We will make our minds up later."
In a sign that he was targeting Labour supporters as well as Tory votes, he added: "The really big hits we are getting are in the north of England and the Labour Party, actually the Conservative Party is holding quite steady in many areas, it's not quite as bad a night for David Cameron as some people have predicted."
Lib Dem Cabinet minister Danny Alexander did not rule out the Liberal Democrats finishing fifth behind the Green Party - but said he expected to retain some MEPs.
He told the BBC: "Some people have been predicting that we will end up with no MEPs at all. I suspect we will end up in the very low single figures."
David Cameron, Ed Miliband and Mr Clegg have all faced criticism over the way they have responded to Ukip's rise and strategists will study the results of the final national ballot ahead of next year's general election as they plan how to tackle a new era of four-party politics.
Eurosceptic parties have prospered across the EU, with o ne of the most significant winners appearing to be France's far-right Front National.
Two polling companies said the party, led by Marine Le Pen, was the outright winner in France with one-quarter of the popular vote.
Tory chairman Grant Shapps told LBC: "W e are seeing results coming in across Europe with a very clear message to the elite in Europe that people want to see change.
"The Conservative Party has long since got that and that is why we want a referendum following renegotiation."
Mr Shapps added that many voters saw this contest as a "free hit election", indicating they could swap their allegiance back to the Tories after delivering a protest by supporting Ukip.