"Hillary Clinton's corruption is on a scale we have never seen before. We must not let her take her criminal scheme into the Oval Office," the Republican nominee said.

Mr Trump stepped up his assault as the latest polls showed him narrowing the gap on Mrs Clinton, both nationally in key battlegrounds states.

Speaking in Phoenix, Arizona, on Saturday, Mr Trump continued his attack, accusing the Justice Department of trying to help Mrs Clinton.

"When the outcome is fixed, when the system is rigged, people lose hope. They stop dreaming. They stop trying," said Mr Trump.

"Hillary Clinton's corruption is corrosive to the soul of our nation and it must be stopped."

A Democrat group has filed a formal complaint with the Department of Justice against James Comey, accusing the FBI director of interfering in the presidential election. The Democratic Coalition Against Trump alleges that Mr Comey is in breach of the Hatch Act, which bars federal employees from political activity.

“It is absolutely absurd that FBI director Comey would support Donald Trump like this with only 11 days to go before the election,” said Scott Dworkin, senior advisor to the Democratic Coalition Against Trump.

“It is an obvious attack from a lifelong Republican who used to serve in the Bush White House, just to undermine her campaign. Comey needs to focus on stopping terrorists and protecting America, not investigating our soon to be president-elect Hillary Clinton.”