Outspoken billionaire Donald Trump took the limelight at a debate between the Republican Party's top 10 presidential candidates on Thursday evening, when he refused to rule out an independent run at the White House.

Tensions were high from the very beginning of the debate, which began with a request from the Fox News hosts for candidates to raise their hands if they would not back the sole final nominee. Trump was the only one to do so.

"I will not make the pledge at this time," said Trump, who has enjoyed an unexpected surge in the polls to be the Republican party candidate for the 2016 presidential election.

Boos were heard from the Cleveland, Ohio, audience. The refusal also led to a fraught exchange with Senator Rand Paul, who said Trump was "already hedging his bets because he's used to buying politicians."

Trump countered with a quip that, as a patron of political causes, he had given Paul "plenty of money."

Former Florida Governor Jeb Bush was placed on the defensive more than once, standing by a statement he made last year that people coming to the US illegally were often doing so as an "act of love" for their families. Bush maintained that many were merely trying to provide for their families, a stance that prompted groans from the audience.

Bush says Iraq war 'a mistake'

As brother of former US President George W. Bush, Jeb Bush surprised some as the conversation turned to Iraq. "It was a mistake," Bush said. "I wouldn't have gone in."

While Trump is on top of the polls, he has been criticized by Bush for having a tone that was too negative and angry, particularly in reference to immigration from Mexico. However, the property mogul appeared to stick to his strategy in the debate. "We don't have time for tone," said Trump, in remarks about Islamist militants. "We've got to get out and get the job done."

The debate touched on a range of issues, with former Arkansas Governor Mike Huckabee saying he would like to see the US Constitution changed to protect unborn children. "It's time we admit the Supreme Court is not the supreme being," he said.

Huckabee has said he would remove funding for the Planned Parenthood organization, which provides abortion services as well as help with contraception.

No experience necessary

Florida Senator Marco Rubio meanwhile claimed executive experience was not necessary for the candidate running for the presidency. The comments came after a comparison was drawn between his track record and that of Florida rival Bush.

"I would add to that this election cannot be a resume competition," Rubio retorted. "This election better be about the future, not the past."

The 10 candidates in the main debate also included Wisconsin Governor Scott Walker, surgeon Ben Carson, Texas Senator Ted Cruz, Ohio Governor John Kasich and New Jersey Governor Chris Christie.

Seventeen candidates in all are in the running, with an earlier debate being held for the bottom seven candidates in polling terms. US commentators and social media broadly declared former Hewlett-Packard Co chief executive Carly Fiorina to have won that round of political sparring.

rc/cmk (AFP, AP, dpa, Reuters)