Hillary Clinton and Donald Trump

Donald Trump and Hillary Clinton on Monday faced off in a presidential debate that could be the most-watched in U.S. history, with the high-stakes showdown expected to attract 100 million viewers.

The 90-minute contest at Hofstra University on Long Island, New York is the first of three planned debates.

It said that the debate would also be one of the last chances for the two candidates to sway voters ahead of the November 8 election.

An opinion poll released showed a dead heat at the national level, with both nominees receiving 46 per cent of likely voters in a Bloomberg survey.

However the presidential election is won state-by-state – especially so-called swing-states such as Ohio, Pennsylvania, North Carolina and Florida and by that measure Clinton still has a slight advantage.

The debate would be moderated by NBC journalist, Lester Holt and will feature questions about the direction of the country, the economy and national security.

Clinton, a Democrat whose political career spans four decades, is widely seen as the better debater.

But Trump, a novice politician who made his name in real estate and reality television, has a bombastic and unpredictable style that analysts say makes him a tricky opponent to go up against.

Anticipation has been building for the head-to-head encounter, with media watchers estimating that more viewers will tune in than for any other previous presidential debate.

The audience for the Clinton-Trump confrontation is expected to surpass the 1980 presidential debate between Jimmy Carter and Ronald Reagan, in which 80 million watched.

The first 2012 debate between President Barack Obama and Mitt Romney had 67 million viewers. (dpa/NAN)