Donald Trump has accused Democrats of being "phony hypocrites" over their opposition to his decision
Sacked FBI director James Comey has said he "will be fine" and told his former colleagues that he "will miss you and the mission deeply".
Mr Comey was fired by President Donald Trump on Tuesday, with the FBI chief reported to have learned of his dismissal from TV news while talking to agents in Los Angeles.
In a letter to agency staff widely reported by US media, Mr Comey told his former colleagues: "I have long believed that a President can fire an FBI Director for any reason, or for no reason at all.
"I'm not going to spend time on the decision or the way it was executed. I hope you won't either. It is done, and I will be fine, although I will miss you and the mission deeply."
He said Americans should see the bureau as a "rock of competence, honesty, and independence" during "times of turbulence".
He added: "It is very hard to leave a group of people who are committed only to doing the right thing. My hope is that you will continue to live our values and the mission of protecting the American people and upholding the Constitution.
"Working with you has been one of the great joys of my life. Thank you for that gift."
The White House has officially cited recommendations from the US attorney general and deputy attorney general for the decision to fire Mr Comey, specifically referencing his handling of the Hillary Clinton email controversy.
In a memo, deputy attorney general Rod Rosenstein argued: "Almost everyone agrees that the Director made serious mistakes; it is one of the few issues that unites people of diverse perspectives."
White House spokesperson Sarah Sanders said President Trump "had been considering letting Director Comey go pretty much since the day he took office".
However, concerns have been raised over the fact that the FBI is also investigating alleged Russian interference in last year's US election - including potential links between Russia and the Trump campaign.
The New York Times reported that the firing came only days after Mr Comey asked for additional resources to "accelerate the bureau’s investigation" into Russia.
Democrats and even some Republicans raised concerns over the dismissal.
Democratic leader Nancy Pelosi said the "sudden and brazen firing" of Mr Comey "raises the ghosts of some of the worst Executive Branch abuses".
Independent Senator Bernie Sanders, meanwhile, suggested: "Trump’s firing of James Comey at this time was absolutely uncalled for and a threat to the rule of law in our country."
More than 120 Democratic politicians called for a special prosecutor to handle the Russia investigation, while a handful of Republicans joined dozens of other Democrats in calling for an independent investigation.
Other Republicans defended President Trump's decision, with House Speaker Paul Ryan telling Fox News: "It obviously was in the president's authority and role to do this. I think he made an important command decision, and that decision that is in his right to do it. And now let's go forward."
Trump himself continued to defend the move on Twitter, accusing Democrats of hypocrisy following their previous criticism of Mr Comey's handling of the email controversy.
Dems have been complaining for months & months about Dir. Comey. Now that he has been fired they PRETEND to be aggrieved. Phony hypocrites!— Donald J. Trump (@realDonaldTrump) May 10, 2017
Meanwhile, the US Intel Committee - which is conducting its own Russia probe - yesterday issued a subpoena for documents from Trump's former national security adviser Michael Flynn.
Flynn resigned after claims he misled the Trump administration over his talks with Russia.
"The Committee first requested these documents in an April 28, 2017 letter to Lieutenant General Flynn, but he declined, through counsel, to cooperate with the Committee’s request," the committee said in a statement.