"Personally, when I heard it, I thought that it was not something that was within the realm of possibility that she would have said it. I said to my people, I don't believe she said it, I think you have to check it because there's no way that she said this," Trump told Fox News.
"She actually did, and she even really doubled up because it was said with such anger and such unbelievable anger, and I think this is the biggest mistake of the political season," the Republican presidential candidate said.
Democrat Clinton and Trump face each other in the November 8 general elections that would elect the 45th US president.
According to several polls, Clinton currently leads Trump, but the gap is tightening rapidly.
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The 68-year-old Clinton had sparked an uproar on Friday at a fundraiser when she described Trump's supporters as "deplorables" who were driven by "racist, sexist, homophobic, xenophobic, Islamaphobic" sentiments.
Trump also raked up similar remarks made by Mitt Romney, Republican nominee in the 2012 presidential election.
"Remember this, you're going to be president, you're president of all the people. You're not president of 50 per cent or 75 per cent, you're president of all people - you're president of everybody," Trump said coming back on Clinton.
"And somehow it has to get out there that it's not a certain group of people that's going to be left behind. Or they've already been left behind with jobs and lots of other things. You have to be president of everybody," he asserted.
"People know it's very interesting - people 18 years ago were making more in real wages, many people. But on average, people 18 years ago were making more than they're making today and they're working harder today. And in many cases they - and to a large extent because of this horrible Obamacare - in many cases they're working two jobs," he said.