Donald Trump has started the Republican presidential debate by describing comedian Rosie O’Donnell as a "fat pig" and a "slob".
The real estate mogul, along with rivals Jeb Bush, Scott Walker and seven others are facing off at the debate in Cleveland, Ohio, which started at 11am AEST today.
At the beginning of the debate, he was asked by moderator Megyn Kelly about some of his earlier comments on women.
“You’ve called women you don’t like 'fat pigs, dogs, slobs, and disgusting animals',” she said, before Trump interrupted: “only Rosie O’Donnell.”
Ms O’Donnell appeared to respond to the comments a short time later on Twitter.
Trump dismissed as "political correctness" a question from Ms Kelly, who asked him to answer charges that he was part of a "war on women."
"What I say is what I say," said Trump, who is leading the sprawling Republican field of 17 candidates.
He drew boos from the audience when he pushed back against Ms Kelly by accusing her of not treating him well.
"Honestly Megyn, if you don't like it, I'm sorry. I've been very nice to you although I could probably maybe not be based on the way you have treated me," Trump said. "But I wouldn't do that."
Debate topics
The debate will give Americans their first look at the major Republican candidates en masse six months before the first nominating contest in Iowa.
Before the main event, seven candidates who rank lower in the polls took part in a separate daytime debate and quickly challenged Trump's conservative credentials, noting he had changed positions on abortion, healthcare and other issues.
But Carly Fiorina, a former business executive and the only woman in the Republican field, acknowledged Trump had tapped into a broad sense of frustration with Washington.
"Whatever your issue, your cause, the festering problem you hoped would be resolved, the political class has failed you," Fiorina said. "That’s what Donald Trump has tapped into."
His rapid rise to the top of the Republican pack in polls has earned him center-stage status in the debate.
Trump's take-no-prisoners style of campaigning, with a penchant for incendiary remarks such as those on the war record of Senator John McCain and on Mexican immigrants, whom he branded as criminals and rapists, has also earned him a level of news coverage that has eclipsed many of his rivals.
When asked today about earlier comments that America needed to "build a wall" to prevent Mexican people crossing into the United States he said he had sparked a conversation.
“If it weren’t for me, you wouldn’t even be talking about illegal immigration," he said, adding: "We need to build a wall…we need to keep illegals out.”
Other leading contenders include Jeb Bush and Florida Senator Marco Rubio.
The Bush camp made clear the former Florida governor intends at the debate to sell his record as a conservative, tax-cutting governor. The Walker camp made similar pledges about the Wisconsin governor's aim.
Republican strategist Katie Packer said the smart strategy is to steer clear of the mud-slinging Trump.
"There's no point in fighting with a pig because you just get dirty and you make the pig mad," she said.
