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."