Trump centre stage in Republican debate

Bombastic, comical, provocative.

Donald Trump, the billionaire leading the Republican primary race, offered the best and worst of himself in the first major debate of the 2016 presidential race.

Flanked by nine rivals who trail him in the polls Trump immediately set himself apart, refusing to rule out running as an independent if he loses the party primary.

He also couldn't promise to back the eventual Republican nominee.

"I will not make the pledge at this time," the improbable frontrunner said, to loud boos and jeers from the crowd.

The remark triggered an angry exchange with Senator Rand Paul, who shouted across the stage that Trump was "already hedging his bets because he's used to buying politicians".

It was an extraordinary start to Republican's quest to choose a flagbearer for the 2016 presidential campaign, only six months ahead of the first primary votes.

By the time the event was over Trump had called US leaders and politicians "stupid", claimed he had given money to most of the candidates on the stage - as well as to top Democrat Hillary Clinton - and said he had no time for "political correctness".

Trump's unapologetic, off-script style offends some, but has set him apart from a packed field of hopefuls furiously trying to garner the same level of attention.

"Donald Trump's hitting a nerve in this country," admitted his rival Ohio Governor John Kasich.

"For people who want to just tune him out, they're making a mistake."

Former Florida governor Jeb Bush acknowledged that the bar is likely higher for him in 2016, being the son and brother of two presidents.

But he insisted once more that he is his own man with his own policies.

"I'm going to have to earn this," Bush said.

Bush accused Trump of using "divisive" language, and warned such verbal sniping will not help Republicans win the White House.

Candidates sought to make an impression on voters - and many aimed at President Barack Obama, Clinton and Trump.

They offered withering attacks on Obama's treatment of the Islamic State group; vowed tougher immigration policy; pledged to toe the conservative line on social issues; and stressed they would shred a nuclear deal with Iran on day one of a Republican presidency.

Immigration is a favourite subject of Trump, who has promised to build a wall between Mexico and the United States to stop illegal immigrants.

He sparked a firestorm and offended many Hispanic voters when he said Mexico was not "sending its best", and that immigrants were bringing drugs and crime to the US.

"If it weren't for me, you wouldn't even be talking about illegal immigration," Trump said on Thursday as the debate began.

"He's saying a lot of the things that everybody thinks and he says them without being embarrassed," said Brad Roller, in a bar in Cleveland after a campaign event for Senator Marco Rubio.

The candidates in Thursday's debate were: Trump, Bush, Paul, Rubio, Kasich, Wisconsin governor Scott Walker, former Arkansas governor Mike Huckabee, neurosurgeon Ben Carson, Senator Ted Cruz and Governor Chris Christie of New Jersey.