Donald Trump has described Hillary Clinton as a "nasty woman" in an interjection towards the end of the third presidential debate in Las Vegas this afternoon.

"Such a nasty woman," Trump said after Clinton made a slight at Trump.

It followed a moment when Trump refused to commit to accepting the result of the election if he were to lose.

Trump said he "will look into it at the time" when asked by moderator Chris Wallace if he would concede if his election is unsuccessful.

"I'll keep you in suspense, okay?" he said.

He said millions of people were registered to vote who should not be, and said it was "crooked" and "rigged" that Clinton was allowed to run.

"She should not be allowed to run. She is guilty of a very, very serious crime," Trump said.

Clinton pointed out Trump makes accusations of rigging when he doesn't win, including when he accused the Emmys of being rigged when he didn't win for The Apprentice .

"Shoulda gotten it," Trump interjected.

Who do you think won the debate? Donald Trump 2152 Hillary Clinton 6692

A post-debate poll from CNN/ORC found viewers overwhelmingly considered Clinton the winner, by a substantial 53-39 margin.

It's a wider lead than her win in the second debate but not as strong as her first debate win.

A YouGov poll showed a narrower 49-39 win for Clinton.

Trump is currently trailing by double-digits in many nationwide polls and is facing a landslide loss unless his campaign can turn things around in the final weeks of the election.

Assault allegations

In perhaps her strongest moment of the debate, Clinton tore into Trump for his alleged indecent assaults against women.

In a lengthy rebuke, Clinton slammed Trump's attitude to women.

"Donald thinks belittling women makes him bigger," Clinton said.

"I don't think there is a woman anywhere who doesn't know what that feels like.

"I think it's really up to all of us to demonstrate who we are. To stand up and be very clear about what we expect from our next president."

Trump dismissed the claims, saying many of the accusations against him were "largely debunked".

"I didn't even apologise to my wife because I didn't do anything," Trump said.

"I think (the accusers) want either fame, or (Clinton's) campaign did it."

Donald Trump and Hillary Clinton. (AFP)

More accusations

Trump then accused Clinton of instigating a riot in Chicago during a Trump rally.

"She's the one and Obama that caused the violence."

Who do you want to win the US election? Donald Trump 1562 Hillary Clinton 5124

"It's so sad when she talks about violence at my rallies when it's her fault."

Trump described The Clinton Foundation as a "criminal enterprise" and slammed Clinton for taking charity donations from Saudi Arabia.

Clinton also attacked Trump for dodging paying taxes.

"We have undocumented immigrants who are paying more federal income tax than a billionaire," Clinton said.

Kremlin interference

In a remarkable moment early in the debate, Clinton said Trump would be a puppet of Vladimir Putin.

"The Russian government has engaged in espionage against Americans," she said.

"Then they have given that information to Wikileaks for the purpose of putting it on the internet."

"It's pretty clear that Vladimir Putin would rather have a puppet in the White House than a president."

Trump said Clinton did not know whether Russia, China or anyone else was responsible for the attacks.

"You're the puppet," Trump retorted.

"I don't know Putin. I never met Putin. This is not my best friend.

"She doesn't like Putin because Putin has outsmarted her at every step of the way."

Clinton and Trump did not shake hands when they entered but instead stayed on separate ends of the stage.

The Supreme Court

Clinton began the debate with an impassioned plea to maintain same-sex marriage and abortion rights as well as tighten campaign finance restrictions when speaking about the Supreme Court.

Trump then brought up criticism he had received from Supreme Court Justice Ruth Bader Ginsberg and then told the audience gun rights were "under siege" and "under such trauma".

The debate covered a topic only briefly covered in previous debates, gun rights.

"We have 33,000 people a year who die from guns. I think we need comprehensive background checks," Clinton said.

"I see no conflict between saving people's lives and the Second Amendment."

Trump said he would appoint Supreme Court justices who would overturn Roe v Wade , the court ruling which legalised abortion nationwide.

Clinton then launched an impassioned defence of a politically contentious topic, late-term partial birth abortion.

"That's not okay with me," Trump replied.

Clinton then challenged Trump to meet women who have had to have late term abortions for health reasons.

Border security

Trump called for strong borders to stop drugs "pouring over" and reasserted his policy of building a wall between the US and Mexico.

"We have some bad hombres here and we're going to get them out," he said.

Clinton replied by saying she did not want to rip families apart, and Trump's "deportation force" would go "school-to-school, home-to-home, business-to-business" to round up undocumented immigrants.

"I voted for border security in the United States Senate," she said.

Clinton then accused Trump of choking when discussing the wall with the Mexican president, then said Trump used undocumented workers to build Trump Tower.

"We either country or we don't. We are a country of laws," Trump said when accusing Clinton of wanting to open borders.

He did not deny accusations he used undocumented workers.

Tax plans

Clinton went on to label Trump's tax plan as "trickle-down economics on steroids".

Trump said Clinton's plan would "double your taxes".

"Our country is stagnant. We've lost our jobs. We've lost our businesses," Trump said.

"There's only one of us on this stage which has shipped jobs to Mexico," Clinton replied.

"Donald has bought Chinese steel and aluminum. So he goes around with crocodile tears because he has gone around and given jobs to Chinese steelworkers and not Americans."

Trump said if Clinton did not want him using Chinese steel, she should have "made it impossible for him to do that".