Debate watchers broke the rules on Monday, cheering for Donald Trump - even though they'd been instructed to stay silent - as the Republican brought up Hillary Clinton's email scandal.

Trump said he'd release his tax returns despite an ongoing audit - but only if Clinton produces the emails she deemed personal and had deleted from her server and email account before she turned over her work product to the State Department.

'I will release my tax returns against my lawyers' wishes when she releases her 33,000 emails that have been deleted,' he stated. 'As soon as she releases them I will release my tax returns and that's against my lawyers, they say don't do it.'

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Debate watchers broke the rules on Monday, cheering for Donald Trump even though they'd been instructed to stay silent as the Republican hit Hillary Clinton for her email scandal

Clinton charged in turn that Trump won't give the public access to his returns 'because there's something he's hiding.'

'Who does he owe money to? He owes you the answers,' Clinton stated.

She again apologized for keeping her basement secret a secret and told voters, 'I made a mistake using a private email.'

'That's for sure,' Trump interjected.

Continuing, Clinton said, 'If I had to do it over again, I would obviously do it differently. But I'm not going to make any excuses. It was a mistake and I take responsibility for that.'

Trump told the former secretary of state, 'That was more than a mistake. That was done purposely.'

That her staff invoked their Fifth Amendment rights so that they're 'not prosecuted,' he said, 'I think its disgraceful.'

'And believe me, this country really thinks its disgraceful,' he added.

Turing back to his taxes, Trump defended himself as a savvy businessman who is 'extremely underleveraged.'

'I have a great company and I have tremendous income,' he stated. 'I say that not in a braggadocios way but it's time that this country has somebody running the country who has an idea about money.'

The country is a 'mess' and owes trillions to foreign lenders because of politicians like Clinton, he contended.

'We don't have the money because it's been squandered on so many of your ideas,' he told his opponent.

Clinton shot back, 'Maybe it's because you haven't paid any federal income tax for a lot of years.'

Trump did not deny the claim - he instead said, 'It would be squandered, too, believe me.'

Trump deflected from his taxes to Clinton's emails after Lester Holt noted that voters cannot know the full extent of Trump's business dealings and where potential conflicts of interest lie because he's yet to make the documents public

Trump's taxes came up as moderator Lester Holt noted that voters cannot know the full extent of Trump's business dealings and where potential conflicts of interest lie because he's yet to make the documents public.

'I don't mind releasing. I'm under a routine audit. It will be released as soon as the audit is complete,' Trump said, 'but you will learn more about Donald Trump by going down to the federal elections where I filed a 104-page essentially financial statement of sorts, the forms that they have.'

His Federal Elections Commission forms contain his income, which is filed at $694 million last year, the businessman said.

Changing the subject to Clinton's emails, he demanded to see the messages she said her team deleted, some of which were subsequently recovered by the FBI.

The audience reacted, and Holt reminded them they were asked to be respectful of the debate process.

'Let me admonish the audience one more time. There was an agreement. We asked you to be silent. It would be helpful for us,' he said.

Debate attendees were silent throughout most of the debate - but Trump's jab at Clinton over her emails had them hooting

Clinton asserted that Trump is the only presidential candidate in the past 40 years not put out his returns.

'You've got to ask yourself, why won't he release his tax returns? And I think there may be a couple of reasons,' she said. 'First, maybe he's not as rich as he says he is. Second, maybe he's not as charitable as he claims to be. Third, we don't know all of his business dealings.'

The Democratic White House candidate speculated that Trump does not perhaps, 'want the American people...to know that he's paid nothing in federal taxes because the only years that anybody has ever seen were a couple of years when had he to turn them over to state authorities when he was trying to get a casino license and they showed he didn't pay any federal income tax.'

Trump retorted, 'That makes me smart.'

In the spin room after the debate, Alabama Senator and Trump surrogate Jeff Sessions told reporters, ‘I don’t know what he meant at all.’

Sessions insisted that federal investigators would have pursued criminal charges against Trump if he'd broken the law.

‘It is essential that Donald Trump and the American taxpayers pay what they’re required to pay under the law. And that’s what he said,' Sessions asserted. 'He’s been audited for years, apparently nobody’s prosecuted him for any kind of violation of his taxes. It’s not because they hadn’t looked at them.'