"Director Comey was very unpopular with most people", Mr Trump claimed, adding that he thought his firing would be a "bipartisan decision".
Trump went on to dismiss as "totally ridiculous" the notion that he himself may have committed a prosecutable - or even impeachable - offence in recent months, as argued by some critics who suspect him of obstructing the FBI's probe.
Former FBI Director James Comey, who had previously led the investigation, was sacked by Trump on May 9.
On Wednesday, the Justice Department appointed former FBI Director Robert Mueller as a special counsel to lead the probe into Russian interference in the 2016 US election.
But Mueller and Comey have been longtime allies dating back to at least 2003 when the men both worked in Washington, D.C., Mueller as FBI Director and Comey as Deputy Attorney General.
President Donald Trump charged yesterday that the stepped-up probe into whether his campaign colluded with Russian Federation to tilt last year's election was dividing the country.
"The entire thing has been a witch hunt", he declared, echoing one of the tweets he'd sent out just after dawn: "This is the single greatest witch hunt of a politician in American history!"More news: In Debate Prep, Hillary Clinton Practiced Dodging Trump's Weird Macho Hugs
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He did not offer any evidence of such acts in his reference to former Democratic President Barack Obama and former presidential candidate Hillary Clinton.
"I think it divides the country", he told a joint press conference with Colombian President Juan Manuel Santos.
Rosenstein, the No. 2 Justice Department official, named Mueller amid mounting pressure in Congress for an independent investigation beyond existing FBI and congressional probes into the Russian Federation issue.
Despite initially opposing appointment of an independent counsel, Republican House Speaker Paul Ryan said Thursday that the development "helps assure people and the Justice Department that they're going to go do their jobs independently and thoroughly, which is what we've called for all along". On Wednesday, the president reportedly interviewed four candidates for Comey's former position at the White House.
"And he said that ever since, the President had been trying to be chummy in a fashion that Comey felt was created to absorb him into Trump's world - to make him part of the team".
Sen. Claire McCaskill, D-Mo., said after an all-senators briefing from Rosenstein that the deputy attorney general already knew Comey would be fired before he wrote a memo that the Trump administration initially used as justification for ousting the Federal Bureau of Investigation chief. He also told network anchors at the White House on Thursday that appointing a special counsel "hurts the country".