JUST IN: Stampede as dangerous substance is found at Hillary Clinton's HQ building

Hillary Clinton's campaign headquarters had to be evacuated on Friday night after a "suspicious substance" was allegedly found inside a letter, it was reported.

Two interns at the Democratic presidential nominee's Manhattan office opened a letter and discovered the substance at around 5pm, according to the New York Post.

The staffers then reportedly took the letter to the Clinton campaign office headquarters in Brooklyn at around 5.30pm, police told the Post.

Four people are believed to have been exposed to the substance, prompting emergency services to evacuate the entire office.

It is not yet known whether the substance was dangerous.