Nigel Farage said mass immigration was making parts of the country appear "unrecognisable" and like "a foreign land" at Ukip's spring conference on Friday.

The speech put immigration at the heart of Ukip's campaign for the European and local elections, which Farage followed with a declaration that he would resign if his party failed to win a seat in parliament in 2015. But he appeared to concede some concerns raised by Ukip about the scale of immigration from Bulgaria and Romania after the lifting of transitional controls in January may have been unfounded. The greatest potential immigration threat now came from the eurozone, Farage claimed.

"In scores of our cities and market towns, this country in a short space of time has frankly become unrecognisable," Farage told his audience in Torquay. "Whether it is the impact on local schools and hospitals, whether it is the fact in many parts of England you don't hear English spoken any more. This is not the kind of community we want to leave to our children and grandchildren."

Asked at his press conference to justify the comments, Farage cited a recent experience on a rush-hour train leaving Charing Cross. "It was a stopper going out and we stopped at London Bridge, New Cross, Hither Green, it was not until we got past Grove Park that I could hear English being audibly spoken in the carriage," he said. "Does that make me feel slightly awkward? Yes it does." Asked why he minded people speaking in foreign languages, he replied: "I don't understand them … I don't feel very comfortable in that situation and I don't think the majority of British people do."

With some commentators tipping Ukip to win the Europeans elections, even though polls have yet to show it overtaking Labour, members arrived in Torquay knowing expectations are high but that the party is still tainted by associations with eccentrics and extremism. Ukip believes its stance on immigration can win votes from all sides of the political spectrum.

Last year Ukip's anti-immigration campaign focused on the impact of transitional controls being lifted from January 2014. Ukip was strongly criticised for issuing campaign leaflets saying 29 million Bulgarians and Romanians would have the right to live, work and draw benefits in the UK from the start of the year. But, after two months, early indications are that the mass influx some predicted has not materialised – David Cameron recently said immigration levels from Bulgaria and Romania were "reasonable" – and in his speech Farage instead raised a fresh immigration concern.

"It isn't directly Romania and Bulgaria I'm necessarily concerned about. What I'm really concerned about is the fact in the eurozone, in the Mediterranean there is no sign or prospect of any significant recovery at all," he said.

"If the eurozone goes as badly over the next few years as I still believe that it will, we face the prospect of the largest migratory wave that has ever come to this country and we have three political parties who are not prepared to do anything about it."

Farage also said that, if Ukip did well in 2014, they should be able to use that as a platform to win some seats in the general election. Asked if he would resign as leader if failed to get a Ukip MP elected, he replied: "Good lord, yes. I will be out the door before you can say Jack Robinson."

He faced a barrage of hostile questions at a press conference about the party's decision to appoint Neil Hamilton, the disgraced cash-for-questions former Tory minister, as Ukip's campaign manager for the 2014 election. Farage insisted that Hamilton was just a "backroom boy", but this was disputed by Hamilton himself who told the BBC's Newsnight that he saw himself as "front-of-house".

In response to questions about Hamilton's role with the party, Farage said: "There are things that went wrong in his career. We all have things in our life that have gone wrong." It was in the past, he insisted.

But Hamilton himself showed no contrition himself when he used his speech to attack the Westminster political class.

He attacked "the deracinated political elite of parasites, the bureaucrats, the Eurocrats, the quangocrats, the expenses-fiddlers, the assorted chancers, living it up at taxpayers' expense". It was Ukip's historic role "to sweep them all away", he said.