Trump takes center stage in prime-time Republican debate

Other candidates try to stand out in crowded field

Bombastic, comical, provocative: Donald Trump, the billionaire leading the Republican primary race, offered the best and worst of himself on Thursday as opponents strained to offer more gravitas in the first major debate of the 2016 election campaign.

Flanked by nine rivals who trail him in the polls, the real estate mogul immediately set himself apart.

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 loses the party primary.

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

It was an extraordinary start to the party's quest to choose a flag-bearer for the 2016 race to succeed President Barack Obama, 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, clashed with Senator Rand Paul and moderators, and said he had no time for "political correctness".

With 17 major Republican candidates in contention, broadcaster Fox News split the debate into two parts, with bottom-tier hopefuls trading barbs in a separate forum ahead of the prime-time event.

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," said his rival, Ohio governor John Kasich, during the main event. "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 to be higher for him in 2016, being the son and brother of two presidents.

But he again insisted that he is his own man with his own policies. "I'm going to have to earn this," Bush said.

He also stood by his earlier remark that immigrants breaking the law to come to the United States did so as an "act of love".

Bush accused Trump of using divisive language, warning that such verbal sniping will not help Republicans to win the White House.

The candidates also focused their ire on former secretary of state Clinton, the likely Democratic nominee.

"If Hillary is the candidate, which I doubt, that would be a dream come true," said neurosurgeon Ben Carson, the only black candidate in the field.

Candidates at both the main event and on the debate undercard sought to make an impression on voters - and many took aim at Obama, Clinton and Trump.

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 scrap a nuclear deal with Iran on Day 1 of a Republican presidency.

"Under President Obama and Secretary Clinton, they're working hard to change the American dream into the European nightmare," said Louisiana governor Bobby Jindal, one of seven candidates on stage for the early forum.

Carly Fiorina, the former Hewlett-Packard chief, accused Trump of cozying up to the Clintons and accused him of flip-flopping.

"Since he has changed his mind on (immigration) amnesty, on healthcare and on abortion, I would just ask what are the principles by which he will govern?" asked Fiorina.