Democrats and other Trump Administration critics have attacked the firing of No. 2 FBI official Andrew McCabe as a politically motivated and vindictive move.
Some Republicans praised US Attorney-General Jeff Sessions for decisive action.
Sessions fired McCabe yesterday, just two days before the FBI official would have been eligible to retire with a full pension, saying an internal Justice Department watchdog found he leaked information and misled investigators.
McCabe, who played a key role in the FBI probes of Hillary Clinton and Russian interference in the 2016 US election, said he believed he was fired for corroborating former FBI Director James Comey's claim that President Donald Trump had tried to pressure Comey into killing the Russian investigation.
Trump described McCabe's firing as a "great day" in a post on Twitter yesterday, prompting a scathing response from John Brennan, a former head of the Central Intelligence Agency during the Obama Administration.
"When the full extent of your venality, moral turpitude, and political corruption becomes known, you will take your rightful place as a disgraced demagogue in the dustbin of history," Brennan wrote.
"You may scapegoat Andy McCabe, but you will not destroy America...America will triumph over you."
Senator Ben Cardin, the top Democrat on the Senate Foreign Relations Committee, said on MSNBC that McCabe's dismissal was "very vindictive".
"It also is a reflection on the administration's disdain for the FBI itself. I just think this is outrageous. The Department of Justice, the FBI, should be independent and this type of political behaviour should be unacceptable," Cardin said.
Democratic Senator Kamala Harris, who serves on the judiciary and intelligence committees, called on Sessions to testify before Congress to determine whether McCabe's firing was a retaliation against those investigating Russian election interference and Moscow's alleged ties to Trump's campaign.
Republican Representative Bob Goodlatte, who heads the US House of Representatives Judiciary Committee, said in a statement that McCabe's actions had "tarnished" the FBI's reputation.
"I applaud Attorney-General Jeff Sessions for taking action and firing former FBI Deputy Director Andrew McCabe prior to his scheduled retirement," Goodlatte said.
Comey has testified that he believes his own firing by Trump in May was a move to undermine the FBI's investigation of possible collusion between the Russians and Trump campaign officials. Trump has denied such ties.
Comey tweeted: "Mr President, the American people will hear my story very soon. And they can judge for themselves who is honourable and who is not."
Comey has a book scheduled for release in April called A Higher Loyalty.
- Reuters, AAP, AP