Donald Trump slammed the FBI as he hailed the firing of a veteran Bureau agent as a "great day for democracy," a move his attorney said he hoped would bring an end to a probe into alleged collusion between the president's campaign and Russia.

"Andrew McCabe FIRED, a great day for the hard working men and women of the FBI - A great day for Democracy. Sanctimonious James Comey was his boss and made McCabe look like a choirboy. He knew all about the lies and corruption going on at the highest levels of the FBI!" President Trump tweeted.

McCabe is a potential key witness in the Russia probe, however the facts that Comey revealed after his dismissal and subsequent testimony, as well as the recent scandal regarding dubious, at best, FISA court warrants against a staffer of the Trump campaign have prompted the President to call out corruption and bias within the agency.

"[The Mueller probe was] based on fraudulent activities and a Fake Dossier paid for by Crooked Hillary and the DNC, and improperly used in FISA COURT for surveillance of my campaign." Tweeted the President.

He also said on Saturday that the "tremendous leaking, lying and corruption at the highest levels of the FBI, Justice & State."

In addition he reiterated long-running criticism of the Robert Mueller investigation, terming it a "witch hunt" due to revelations that certain members of the investigation team were heavily biased against President Trump, and saying that it "should never have been started in that there was no collusion and there was no crime."

Earlier, President Trump's personal attorney, John Dowd, told the Daily Beast that he hoped Deputy Attorney General Rod Rosenstein would follow the lead of the FBI Office of Professional Responsibility and "bring an end to alleged Russia Collusion investigation manufactured by McCabe's boss James Comey based upon a fraudulent and corrupt Dossier."

Dowd told the Daily Beast he was speaking for the president, but in a subsequent statement he said he had been "speaking for myself, not the president."

McCabe kept memos of his interactions with Trump, just like Comey, US media reported Saturday, adding that the documents could bolster his version of events; however the President has disputed this, saing that he "Spent very little time with Andrew McCabe, but he never took notes when he was with me. I don't believe he made memos except to help his own agenda, probably at a later date. Same with lying James Comey. Can we call them Fake Memos?"

The Justice Department said an internal investigation had found that McCabe made unauthorized disclosures to the media, and had not been fully honest "on multiple occasions" with the department's inspector general.

"The FBI expects every employee to adhere to the highest standards of honesty, integrity and accountability," Attorney General Jeff Sessions said in a statement.

Lack of candor under oath is a firing offense at the FBI, but the politically-charged context of the move raised questions among McCabe's backers.

President Trump, in an early afternoon tweet, belittled the media for its coverage of the story, saying: "The Fake News is beside themselves that McCabe was caught, called out and fired..."

Details of the inspector general's probe were not made public, but it involved the FBI's handling of the 2016 investigation into President Trump's election rival, Democrat Hillary Clinton.

President Trump has repeatedly accused McCabe and Comey of protecting Clinton from prosecution, including over her misuse of a private email server while she was secretary of state.