It was not clear which inquiries the President was referring to, but the FBI is currently looking into alleged links between the Trump campaign team and Russia during the 2016 US election.
Mr Trump claimed leading investigators were biased against his Republican party.
He tweeted: "The top Leadership and Investigators of the FBI and the Justice Department have politicized the sacred investigative process in favor of Democrats and against Republicans - something which would have been unthinkable just a short time ago."
He added: "Rank & File are great people!"
Mr Trump's post came as he was expected to approve the release of a confidential Republican-drafted memo alleging abuse of authority in the FBI''s inquiry into his election campaign.
More follows...