Donald Trump said if he was elected president of the United States he would instruct the attorney general to "investigate" Hillary Clinton to put her behind bars.
Speaking at the second presidential debate in St Louis, he told the Democrat in front of millions of viewers that she should be "ashamed of herself" for deleting 33,000 emails.
"I'm going to instruct my attorney general to get a special prosecutor to look into your situation," he said.
"There have never been so many lies."
Ms Clinton responded that her office had turned over 35,000 emails.
"That was a mistake and I take responsibility for using a personal email account," she said.
"I'm not making excuses," she added.
Mr Trump interrupted several times to say: "You would be in jail".
He also accused the moderators from CNN of not focusing on the subject of Ms Clinton's emails.
She accused Mr Trump in return of "diversion tactics" to avoid talking about his own campaign.
Ms Clinton was investigated for using her personal and unsecured email server during her tenure as secretary of state. FBI director James Comey accused Ms Clinton of being "careless" but did not recommend any criminal charges.
The attorney general's office followed this recommendation and Ms Clinton was cleared of any wrongdoing.
