Bombastic, comical, provocative: Donald Trump, the billionaire leading the Republican primary race, offered the best and worst of himself Thursday as opponents strained to offer more gravitas in the first major debate of the2016campaign.

Flanked by nine rivals who trail him in the polls, the real estate mogul immediately set himself apart when he was the only candidate on stage to refuse to pledge that he would back the Republican nominee and not run for president as an independent if he losesthe party primary.

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

It was an extraordinary start to

the party's quest to choose a flagbearer for the 2016 race to succeed President Barack Obama, only six months ahead of thefirst 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, clashed with Senator Rand Paul and moderators, 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

Despite the Trump show, candidates undercard sought to make an impression on voters - and many aimed at Obama, Clinton andTrump.

They offered withering attacks on Obama's handling 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.

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.

The candidates in Thursday's main debate were: Trump, Bush, Wisconsin governor Scott Walker, former Arkansas governor Mike Huckabee, Carson, libertarian Kentucky senator Rand Paul, Kasich, Florida senator Marco Rubio, Senator Ted Cruz and Governor Chris Christie ofNewJersey.