Donald Trump has refused to say if he would accept the result of the US presidential election if he loses, as he clashed with Hillary Clinton in their final head-to-head debate.

The candidates squared off at the University of Nevada in Las Vegas for the last time before Americans go to the polls on November 8.

Republican Mr Trump was questioned about his claims that the election will be rigged in favour of his Democratic rival, after he tweeted that "there was large-scale voter fraud happening on and before election day".

Asked by moderator Chris Wallace if he would accept the election result, the Republican nominee replied: "I will look at it at the time."

Mr Trump said the media was "so dishonest" and had "poisoned the minds of voters".

Of Mrs Clinton, he added: "She shouldn't be allowed to run. It's crooked. She's guilty of a very serious crime.

"Just in that respect I say it's rigged. She should never have been allowed to run for the presidency based on what she did with emails and so many other things."

Asked again if he would accept the results, Mr Trump replied: "I will tell you at the time. I'll keep you in suspense."

Mrs Clinton said Mr Trump's remarks were "horrifying".

"Every time Donald thinks things are not going in his direction, he claims whatever it is is rigged against him," she said.

"There was even at time when he didn't get an Emmy for his TV programme three years in a row and he started tweeting even the Emmys were rigged against him.

"This is a mindset. This is how Donald thinks. It is funny but it is also really troubling - that is not the way our democracy works."

Mrs Clinton branded Mr Trump a "puppet" of Vladimir Putin after he claimed the Russian president had "no respect" for her.

"I don't know Putin," Mr Trump said. "He nice things about me. If we got along well, that would be good," he said.

"He has no respect for her. He has no respect for our president.

"We're in very serious trouble. We have a country with tremendous numbers of nuclear warheads ... 1,800 nuclear warheads and she's playing chicken.

"From everything I see, Putin has no respect for this person."

Mr Clinton responded: "That's because he'd rather have a puppet as president of the United States".

Mr Trump replied: "No puppet! You're the puppet."

Mr Trump described claims he sexually assaulted a number of women as "totally false" and accused Mrs Clinton's campaign team of creating the allegations.

"I didn't even apologise to my wife who is sitting right here because I didn't even do anything," he added.

"These women ... I think they either want fame or her campaign did it. And I think it's her campaign."

Mrs Clinton said: "Donald thinks belittling women makes him bigger. He goes after their dignity and self-worth."

During the debate, Mr Trump aimed a number of personal attacks on Mrs Clinton, describing her as a "nasty woman".

He added: "The only thing you have over me is experience but it's bad experience.

"If you became president this country is going to be in some mess."

Mr Trump said his desire to build a wall on the US-Mexico border was to keep out "bad hombres".

He said: "One of my first acts will be to get all of the drug lords - we have some bad, bad people in this country that have to go out ... We have some bad hombres here and we are going to get them out.

The two candidates also clashed over the rights of women to have abortion.

Mr Trump said he planned to appoint "pro-life judges" to the Supreme Court and allow individual states to determine whether abortion should be legal.

"If you go with what Hillary is saying you can take the baby and rip the baby out of the womb of the mother just prior to the birth of the baby," he said.

"You can can say that's OK, and Hillary can say that's OK, but that's not OK with me.

"Based on what she's saying ... you can take the baby and rip the baby out of the womb in the ninth month, on the final day, and that's unacceptable."

Mrs Clinton responded: "That's not what happens in these cases and just using that kind of scare rhetoric is just terribly unfortunate."

US president Barack Obama's half brother Malik was in the debate audience as a guest of Mr Trump. He told the New York Post the Republican "can make America great again".

Hours before the debate, several taco vans lined up outside the Trump Hotel near the Las Vegas Strip in a protest over the Republican candidate.