Donald Trump shunned Thursday night's debate of the Republican presidential candidates hosted by Fox News and instead filled an auditorium a few miles down the road, trying to prove his widespread support only days before Iowa kicks off the nominating voting process.

Trump, with just one day's notice on a weeknight, was able to fill to capacity a hall that holds 700.

He was also able to garner a tremendous amount of attention without having to share much of the spotlight. Cable news networks CNN and MSNBC provided extensive coverage of Trump's event.

In deciding to hold a competing event, Trump said the gathering would be to benefit veterans and he welcomed his rivals to attend. Former Arkansas Governor Mike Huckabee and former US Senator Rick Santorum of Pennsylvania joined Trump after participating in the undercard debate, which was held for the Republican candidates at the bottom of the polls.

Trump's decision to skip the debate, which he said was because he would not be treated fairly by Fox News anchor and debate moderator Megyn Kelly and then was mocked in statements by Fox News officials, was sharply criticised by his opponents.

Trump told CNN that someone from Fox News called him to apologise for the remarks made regarding his participation in the debate. "We need people who are going to stand up for themselves and the country," he said. "I was treated very badly and very unfairly by Fox."

There are risks in holding his own event for Trump, who several recent polls have shown barely leading US Senator Ted Cruz.

If Trump wins the Iowa caucuses, the move will be lauded as proof that he has built a movement capable of circumventing establishment media. If he places second, the skipped debate will be blamed as a fatal tactical error that allowed opponents to paint him as weak in the fact of tough questions.

Supporters and some curious onlookers waited in the sub-freezing cold in a line that wrapped around the building and down a block.

Trump's campaign erected a larger Jumbotron outside the auditorium to allow an overflow crowd to watch his remarks.

Before Trump took the stage, some of his well-known supporters spoke. Lynnette "Diamond" Hardaway and Rochelle "Silk" Richardson, two women with a large online video blog following, urged the crowd to back Trump.

Randy Bowling, a Trump supporter from Ottumwa, Iowa, said some of his friends who are undecided in the Republican contest said Trump's decision to not participate in the debate raised doubts about supporting him.

"We have mixed emotions," Bowling said. "We caught a lot of flack from our friends who are on the fence."

Steven Doran, 19, was one of the many students and other curious area residents who attended the event with no plans to ultimately support Trump. Doran plans to participate in the Democratic caucus.

"The spectacle," Doran said when asked why he was there. "I've never seen Trump in person."