The announcement apparently took the White House by surprise, with Trump only being informed of it after Deputy Attorney General Rod Rosenstein had signed the order.
Comey had been leading an FBI investigation into allegations of Russian meddling in the 2016 US presidential election and possible collusion with Trump's campaign.
Russian President Vladimir Putin denied Tuesday that Russia interfered in last year's presidential election in the United States, describing the allegations as "simply rumors" that are being leveraged for political reasons in the United States.
President Barack Obama on Tuesday returned to Chicago - where he gave his victory speech in 2008 - to deliver an emotional farewell speech in front of thousands of Americans, calling on the people to defend their democracy.
