Nigel Farage put concerns about immigration at the centre of his pitch to voters ahead of the European Union referendum.
The Ukip leader said June 23 could be "independence day" if the country voted to sever ties with Brussels, giving the UK back control over its laws and borders.
He said the true scale of immigration would be a "shock" if official figures properly revealed it and challenged Home Secretary Theresa May to a televised debate ahead of the in/out vote.
In a speech at his party's spring conference in Llandudno he warned that the migration crisis would get worse if the country remained in the EU - and claimed it could leave the UK vulnerable to a Paris-style terror attack, or a repeat of the sexual harassment seen in Cologne.
Brandishing his "European Union" British passport, Mr Farage hit out at free movement rules.
Hailing the fact a referendum was taking place as a victory for Ukip, he said: "Let's make June 23 2016 independence day."
But the row between the rival Leave camps cast its shadow over the Ukip gathering, with Mr Farage's support for Grassroots Out and Leave.EU at odds with his sole MP Douglas Carswell and former deputy chairman Suzanne Evans, who back Vote Leave.
At a Vote Leave fringe event Ms Evans, who was sacked as deputy chairman just days before the conference, highlighted research suggesting that Mr Farage was one of the "least trusted voices" on Europe.
Mr Carswell was also at the event, but party sources dismissed as "tosh" reports that he could face being thrown out of Ukip in order to boost Grassroots Out's bid for the Electoral Commission's official designation in the referendum campaign.
Mr Farage said in his conference speech: "It is true that there have been some difficulties on our side of the argument and I have been disappointed and worried that the Vote Leave organisation have not wanted to work with anybody else or to merge with anybody else."
He warned that expansion of the EU, potentially to Bosnia and Turkey, would add to concerns over immigration.
He repeated his claim that National Insurance numbers issued to migrants were far higher than the number of people covered by official migration statistics.
"I do not believe that we are being told the truth about the number of people coming to this country. I believe that the true figures actually would shock us," he said.
"Mass migration into Britain on this scale is not good for our country.
"It is not good for our quality of life, it is not good for social cohesion in our society, and our population inexorably headed towards 70 million or 75 million will not make this a better, richer or happier place to be.
"But as EU members there is nothing we can do about it."
Mr Farage said the possibility of expansion to Turkey, allied to the "desperate problems" in the eurozone "if we remain members of the EU it is a perfectly reasonable, sane thing to say that our migration crisis will get worse".
He added: "Surely one of the first duties of the British Government should be to do everything within their power to protect our people from the horrors that we saw in Paris and the indignities that we saw in Cologne.
"The best and the safest way for us to attempt to prevent such things is to leave the EU and to take back control of our borders."
Mr Farage told activists: "We have forced our political class into giving us this referendum that they never wanted us to have.
"I shall die a happy man, whenever that is, if we win that referendum on June 23. I believe we can do it, I believe we will do it and I will give it everything I have got between now and that day to make sure that we do win."
Stressing the need for a cross-party effort to secure a Brexit, he said: "We cannot and we will not win this referendum purely from the centre-right of politics in this country."
Mr Farage raised doubts about the legal basis of David Cameron's deal with EU leaders and dismissed fears about trading relations with the rest of the EU after a vote to leave, claiming the draw of the UK market would secure a favourable arrangement.
"It is the customer who is king and we clearly are the customer," he said.
Ukip's gathering in Llandudno came ahead of the May 5 elections to the National Assembly for Wales and Mr Farage said he hoped the party would "do very well and make a breakthrough" in Cardiff Bay.
He said he expected Ukip to win seats in Wales, the Scottish Parliament, Northern Ireland and the London Assembly in May's votes.
"Ukip will win seats in all four of the assemblies and parliaments, and in doing that we will be the only party in the UK that can do it," he said.
Mr Farage said the June referendum would benefit his party as it campaigned for May's elections.
"It's actually going to be quite difficult for these contests to take place without the shadow of the referendum hanging incredibly heavily," he told reporters at a pub near the conference venue as he prepared to eat a lunch of liver and onions.
"That probably helps us, I suppose, if I was being a bit cynical about it, but you just can't avoid it.
"This referendum is a massive global political event and it will overshadow many of the elections."
Major Ukip donor Arron Banks, who also funds the Leave.EU campaign and GO, has suggested that the party may not continue in its current form after the referendum.
But Mr Farage suggested that support could grow even after the referendum.
He said: "I was told that when Cameron gave the Bloomberg Speech - that's it, it's all over, the fox has been shot. It's been shot it must be extinct by now. Look, I have no idea what comes after June.
"This referendum could be a seismic event in British politics and it could change things none of us can even predict right now.
"Who would have thought the SNP would lose the referendum, and they skyrocket?"
Asked if he would follow the example of SNP former leader Alex Salmond and stand down if he lost the referendum, Mr Farage said: "I've no idea what I'm going to do after June 23, I've no idea at all. It doesn't matter.
"I will pack it up in 2035. I've no idea, it's a ridiculous question isn't it?"
He defended his decision to share a GO platform with Respect's George Galloway. "George Galloway will speak to the Muslim community in a way that no other British Eurosceptic can do, which is significant," he said.
"And Boris will speak to a different community. This idea that one person must lead and dominate is just nonsense."
He added: "To win a referendum you need a variety of different messengers with different messages. It's a huge mistake to think one person can lead this, absolutely crackers.
"Gerry Gunster, who has won 28 out of 30 referendums in America, says actually a referendum needs several faces and several voices because they will appeal to different audiences.
"The audience that I want to appeal to are primarily non-voters."
Setting out the reason behind his strategy of focusing on migration he said: "We have got a big 10,000 people poll done in early December, I'm quite happy to share the information with you: 60% of the undecideds have said that the cost of EU membership and the implications of open door immigration are the issues upon which they could swing their vote.
"Why do you think this referendum is June 23 and not September 23? Because the Prime Minister knows that a worsening migrant crisis makes his chance of winning much, much less."