WASHINGTON (Reuters) - U.S. Republican presidential candidate Donald Trump insisted on Sunday he was "100 percent right" when he said he saw thousands of Muslims in Jersey City, New Jersey, cheering the 9/11 attacks on the World Trade Center, even though fact-checkers have debunked his assertion.

In a phone interview on NBC's "Meet the Press," Trump said he has heard from "hundreds of people that agree" that there were televised Muslim celebrations of the Sept. 11, 2001, attacks, which he used as evidence to show his remarks were true.

"I saw it. So many people saw it," said Trump, who, in the race for the November 2016 election, has been among the most vocal of the Republican candidates in expressing skepticism about Muslims in the United States. "So, why would I take it back? I'm not going to take it back." 

When NBC anchor Chuck Todd suggested the people Trump heard from are supporters and might want to agree with him, Trump interrupted to note the "huge Muslim population" New Jersey has.

"Why wouldn't it have taken place?" he said of a celebration there. "I've had hundreds of people call in and tweet in on Twitter, saying that they saw it and I was 100 percent right."

Trump came under fire last week after saying at a rally and on television that he had seen thousands of people in Jersey City cheer the collapse of the twin towers in Manhattan. 

His remarks have since been disputed by public officials and fellow Republican candidate New Jersey Governor Chris Christie, who said: "I think if it had happened, I would remember it."

The controversy could be starting to hurt Trump, who has dropped 12 percentage points in the polls in the past five days, according to Reuters/Ipsos opinion polling of Republican voters nationally. He fell from 43 percent to 31 percent, though he continues to hold a wide lead over his competitors. 

When his remarks were revisited on NBC, Trump said: "All over the world - forget about New Jersey for a second - all over the world, it was reported that Muslims were celebrating the fall of the World Trade Center."

But he quickly rejected Todd's assertion that "this didn't happen in New Jersey."

"It did happen in New Jersey," Trump said.

"I have a very good memory, I'll tell you," he said. "I saw it somewhere on television many years ago. And I never forgot it."