Trump then reportedly told Russian officials in the White House that firing Comey had relieved "great pressure" on him.
"I just fired the head of the F.B.I. He was crazy, a real nut job," Mr. Trump said, according to a document summarising the meeting, which was read to The New York Times by a US official. "I faced great pressure because of Russia. That's taken off."
Trump also claimed he was the victim of a "witch-hunt", "the single greatest witch-hunt of a politician in American history".
The White House said Comey's firing was unrelated to the FBI's Russia investigation, but officials didn't deny that Trump had been critical of Comey to Russian officials.
Comey's father, a lifelong Republican, said that he refrained from voting for any presidential candidate.
"I just couldn't vote for Trump," he told The Bergen Record.
Comey's father said his son never told him he had been fired by the President. Instead, he learned of his son's axing through media reports.
"He and I have an unwritten secret agreement that I don't talk about his job," he said.
"It's just a father-son relationship. We never talk about what he does. I read it in the papers."
Republicans criticised Comey last July when he announced that he would not recommend charges for Hillary Clinton over her use of a private email server while secretary of state.
Then, in late October - with the election looming - Comey announced in a bombshell memo that his agency would reopen its Clinton probe.
Democrats blasted Comey for his decision and accused him of contributing to Trump's narrow victory over Clinton.
