Hillary Clinton and Donald Trump face off Monday in one of the most consequential presidential debates in modern US history with up to 100 million viewers set to tune in.

The stakes could not be higher: the first woman to win the White House nomination for a major US party, against a New York tycoon turned reality star who has upended the political establishment.

Commentators call it a clash of the titans: a 90-minute test of endurance between Clinton the Democrat, arguably the most experienced US presidential candidate in history, against Trump the Republican, perhaps the least experienced White House nominee for a major party.

The outcome could shape the last six-week stint of an election that has deeply polarized the country and left Trump fighting to overcome allegations of bigotry and sexism.

Wall Street stocks tumbled Monday in anticipation of the first of three live, televised debates before the November 8 election, with the polls locked in a virtual dead heat between the major-party candidates.

A decisive win for Clinton could see her pull ahead. A strong performance from Trump could keep the vote competitive or even possibly see him pull out in front.

Hosted at Hofstra University on Long Island, a mere 60-minute drive from Manhattan and chaired by NBC Nightly News anchor Lester Holt, the debate may attract an audience closer to the size of the Super Bowl when it kicks off at 9:00 pm (0100 GMT Tuesday), beamed live around the country.

The questions will revolve around three themes: "America’s direction, achieving prosperity, and securing America."

Both candidates, both the most disliked US presidential rivals in contemporary history, have spent days furiously prepping and honing which strategy they believe will inflict maximum damage on the other.

Geo News will go live with real-time Urdu translation when the debate begins at 6:00am PST.