More than 100 million people have already cast their ballots in early voting, putting the country on course for its highest turnout in a century.

Heavy-duty barricades went up Monday afternoon, blocking off the ellipse, a large green area south of the White House, and Lafayette Square to the north.

US President Donald Trump is already threatening to take legal action in a bid to stop the postal ballots from being counted in key battleground states.

The Trump campaign had been planning to pack his Washington hotel to the rafters with supporters hoping to celebrate his re-election.