Nigel Farage has put the issue of immigration at the centre of his argument for the UK to leave the European Union.
Speaking at UKIP's Spring conference in Llandudno, he said the 23 June referendum could be "independence day" if the UK voted to leave the union.
He said he doubted official figures "told the truth" about immigration.
Mr Farage also said he was optimistic his party would "make a breakthrough" in June's Welsh assembly elections.
Elections will be held in Scotland, Northern Ireland and London, as well as in Wales, on 5 May and the UKIP leader told the conference he was "pretty optimistic" the party was going to do "very well".
Mr Farage hailed the fact that an in-out referendum was taking place as a victory for his party, and challenged Home Secretary Theresa May to a televised debate on immigration ahead of the vote.
"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 told the conference.
"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 70m or 75m will not make this a better, richer or happier place to be.
"But as EU members there is nothing we can do about it."
He warned that expansion of EU membership - potentially to Bosnia and Turkey - would add to concerns over immigration, saying: "If we remain members of the EU it is a perfectly reasonable, sane thing to say that our migration crisis will get worse."
And he said remaining inside the union could leave the UK vulnerable to a Paris-style terror attack, or a repeat of the sexual harassment reported in Cologne.
"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."
It comes as Mr Farage dismissed a warning from G20 ministers about the UK leaving the EU as "no surprise" and "mates helping each other out".
Finance ministers from the world's leading economies had warned of a "shock" to the global economy if the UK votes to leave.
"I'm not surprised that big government gets together to support David Cameron," Mr Farage said.
"This is big banks, big business, big government all scratching each other's backs. I don't think that impresses voters."