Hillary Clinton and Donald Trump did not shake hands at the start of the second US presidential debate as it got under way at Washington University in St Louis, Missouri.
The debate is a town hall format with invited audience members asking the two candidates questions.
The first 20 minutes of the 90-minute scheduled debate was dominated by personal attacks.
The first question put to the candidates was if they thought they were modelling appropriate and positive behaviour for children.
In response Mrs Clinton said she has a positive and optimistic view about what Americans can do together and said she will work with every American, and wants to heal the country and bring it together.
Mr Trump said he began the campaign because he was tired of seeing "such foolish things" happen to the US.
He said he wanted to make inner-cities better for African-Americans and Latinos.
In reference to the recent recording that came to light where Mr Trump was heard making lewd remarks about women in 2005, Mr Trump said "this was locker room talk" and "I'm not proud of it".
He said he is "very embarrassed" by the tape but he "will knock the hell out of" Islamic State. He did say he had great respect for women and he has not kissed or groped women without their permission.
Mrs Clinton said with prior Republican nominees for president she disagreed with them but never questioned their fitness to serve but with Mr Trump it is different.
She said Mr Trump's comments showed what he thinks about women and does to women and it is clear the video that has come to light represents exactly who Mr Trump is.
Mr Trump said: "don't tell me about words" and former president Bill Clinton was "impeached" and lost legal license for his actions.
He went on to say Mrs Clinton should "be ashamed of herself" about how she used a personal email server while secretary of state.
Mrs Clinton said it is important to point out that after a year-long investigation there is no evidence of hacking of her private server and no classified material ended up in the wrong hands from her email server.
Mr Trump said Mrs Clinton was "lying again" about the emails.
Mr Trump threatened to prosecute Mrs Clinton if he wins the election on 8 November.
"If I win, I'm going to instruct the attorney general to get a special prosecutor to look into your situation because there's never been so many lies, so much deception," Mr Trump told Mrs Clinton.
His Democratic rival responded: "It's just awfully good that someone with the temperament of Donald Trump is not in charge of the law of our country," prompting the Republican to fire back: "Because you'd be in jail."
Mr Trump brought up Mrs Clinton’s rival for the Democratic nomination, Bernie Sanders saying he never had a chance in the process and he was surprised to see him "sign on with the devil" and support Mrs Clinton.
Clinton vows to investigate Russia for war crimes in Syria
Mrs Clinton backed the establishment of safe zones in Syria, along with efforts to investigate Russia for war crimes committed in support of President Bashar al-Assad.
Mrs Clinton said she supported efforts to probe "war crimes committed by the Syrians and the Russians and try to hold them accountable."
She also accused Russia of trying to tilt the presidential election in Mr Trump's favour with a series of email hacks.
In response, Mr Trump appeared to reject findings from the intelligence community that Russia was behind the hacks, saying "maybe there is no hacking."
Mr Trump denied having loans with Russia and repeated his promise to release his tax returns as soon as an audit is completed.
Trump Muslim ban a 'gift to ISIS'
Mr Trump and Mrs Clinton differed sharply on the issue of policy towards Muslims during the debate, with Mrs Clinton accusing Mr Trump of giving a "gift to ISIS" by suggesting Muslims be banned from entering the United States.
Clinton accusers back Trump ahead of debate
The tone for the night was set when earlier Mr Trump went on the offensive holding a news conference with a panel of women who accused Bill Clinton of sexual harassment and misconduct.
"These four very courageous women have asked to be here," Mr Trump said at the news conference.
Seated beside four women - including Juanita Broaddrick, Paula Jones, Kathleen Willey and Kathy Shelton -Mr Trump addressed viewers ahead of the debate, making an issue of Bill Clinton's own sexual history as the GOP nominee faces a mass defection from within his own party after being caught bragging about sexual assault.
"These four very courageous women have asked to be here, and it was our honour to help them," Mr Trump said of the women.
Juanita Broaddrick, who in 1999 accused Bill Clinton of raping her decades earlier, accused Trump's attackers of hypocrisy.
"I tweeted recently and Mr Trump retweeted it that actions speak louder than words. Mr Trump may have said some bad words but Bill Clinton raped me and Hillary Clinton threatened me. I don't think there's any comparison," she said.
Paula Jones McFadden, who in 1994 accused then-president Bill Clinton of sexual harassment, said she was there to support Mr Trump because he is going to make America great again.
"And I think everybody else should vote for him. And I think they should all look at the fact that he's a good person, he's not what other people have been saying he's being, like Hillary. So think about that."
Bill Clinton was never charged in any of the cases, and he settled a sexual harassment suit with one of the women, Paula Jones, for $850,000 with no apology or admission of guilt.
While Mr Clinton was president in 1998, the Republican-led House of Representative voted to approve articles of impeachment accusing him of perjury for misleading a grand jury about the nature of his relationship with former White House intern Monica Lewinsky.
Mr Trump, the embattled Republican nominee, sought to deflect attention from a scandal over lewd remarks he made about women in 2005 that came to light on Friday.
His running mate Mike Pence criticised him and more than a dozen prominent Republicans withdrew support and urged him to drop out.
The 2005 video of Mr Trump talking on an open microphone showed the then-reality TV star speaking openly about groping women and trying to seduce a married woman.
The video was taped only months after Mr Trump married his third wife, Melania.
The backlash against the video was swift and widespread.
Mr Trump quickly moved to conduct damage control in a video statement today in which he declared himself a changed man and attempted to shift the focus to his opponent, Democrat Hillary Clinton.
He threatened, again, to focus his attacks on the infidelities of her husband, former president Bill Clinton, saying he would talk more about the pasts of both Clintons with only a month until the 8 November election.
Mrs Clinton's campaign says it is "not surprised" to see Mr Trump's "destructive race to the bottom," and called the meeting with accusers of Bill Clinton a "stunt".