Hillary Clinton's campaign headquarters in Brooklyn have been given the all-clear after a white substance delivered through the mail was found to be not hazardous, said authorities.

The substance was first discovered by two interns at a campaign office in Manhattan at around 5pm local time and was transported to Brooklyn Heights, triggering the evacuation of the 11th floor, a New York Police Department spokesman said.

A total of four people were exposed to the substance, police said, though there have been no injuries or reported illnesses.

Police are testing the substance and investigating the identity of the sender.

Ms Clinton was not in New York at the time of the evacuation.

The former Secretary of State was at a community college in Cleveland, Ohio, as part of a campaign tour.

The news comes after a local GOP office in North Carolina was firebombed last weekend. No one was in the building at the time.

"Suspicious" white powder was also sent via mail to the Manhattan home of Donald Trump's son, Eric Trump, earlier this year.

New polls show Ms Clinton is closing in on Donald Trump in the key battleground state — meaning her campaign has glimpsed the possibility of sweeping to victory across all the traditional swing states.

The evacuation of her office comes after she claimed her rival was a threat to American democracy for not vowing to honour the results of the upcoming Presidential election.

Ms Clinton said she had visited countries whose leaders jailed their political opponents and refused to accept the results of elections they did not win.

“We know in our country the difference between leadership and dictatorship,” she said.

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If Mr Trump refused to concede defeat, it would enormously complicate Ms Clinton's crucial early weeks in office.