Donald Trump has refused to say that he will accept the result of the United States presidential election.
When pressed by the moderator of the third and final debate to confirm that he will accept the outcome of the vote, he said: "I will look at it at the time.
"I will keep you in suspense."
Hillary Clinton described his comment as "horrifying" and that it was extraordinary that the nominee of a major party would say such a thing.
The question related to Mr Trump's recent insistent claims that the election is rigged.
Hillary Clinton then said that she agreed with the assessment of her campaign chairman that Mr Trump was the most "dangerous person" to run for the American presidency in modern history.
Before those dramatic moments, the Las Vegas debate had been ill-tempered but comparatively substantive, with the candidates disagreeing and actually debating over issues like Russia, abortion, immigration and foreign policy.
In the most drawn out confrontation of the evening, Mrs Clinton challenged her rival to admit to and condemn Russia's suspected involvement in hacking emails and trying to interfere with the US elections.
She implied that Mr Trump is connected to Russian president Vladimir Putin.
She said Mr Putin would prefer Donald Trump to win because "he would rather have a puppet for a president".
The Republican shot back: "You're the puppet.
"She has been outsmarted and outplayed."
Hillary Clinton greets supporters, lower left, as Donald Trump speaks with his family on stage | Photo: PA Images
The pair also clashed over immigration, with Mr Trump accusing Hillary Clinton of wanting open borders.
Mr Trump promised again to build a wall on the border with Mexico, because "we have some bad hombres here and we are going to get them out".
He also claimed that by appointing pro-life judges to the Supreme Court, the law enshrining a woman's right to abortion would be overturned "automatically".
The candidates did not shake hands before or after the debate, a public display of the rancour between them.
Mr Trump accused the "crooked" Clinton camp of inciting violence at some of his rallies and of orchestrating the recent chorus of allegations that he groped and kissed women without consent.
Mrs Clinton said: "Donald thinks belittling women makes him bigger.
"He never apologises for anything."
The audience at the Thomas and Mack arena at the University of Las Vegas included former Republican Vice Presidential nominee Sarah Palin, who had been invited by the Trump campaign.
In a controversial move, the Trump campaign also invited President Barack Obama's half brother Malik.
Earlier in the year, Malik Obama announced he would be voting for Donald Trump.
The debate comes after the worst two weeks of Donald Trump's campaign.
Hillary Clinton has been pulling away in both the national polls and also in most of the crucial swing states.
Now, even reliably Republican states like Arizona and Georgia could be up for grabs.
This debate represented Hillary Clinton and Donald Trump's last chance to reach a massive TV audience of American voters.
There are now just 19 days until America goes to the polls.