"I don't buy this stuff about him being mentally incompetent or early stages of dementia", Comey said of Trump, according to a transcript from ABC.
He said he thinks it's possible the Russians have something that could compromise Trump.
Comey, fired by Trump in May previous year, was anxious the president may be open to blackmail by Russian Federation given claims he was present when prostitutes urinated on each other during a 2013 Moscow visit. That's not a perspective, that's not a view, that's just a lie.
He also suggested Comey should be imprisoned, saying, "how come he gave up Classified Information (jail), why did he lie to Congress (jail)".
"I'm sorry that I caused all kinds of confusion and pain with the way I described her conduct that led people into all kinds of side roads", Comey said. Mr. Comey said he would have.
Comey added that Trump treated women like they were "pieces of meat", BBC reported.
Comey added that the press conference was his "way to both convey that and to capture that it was more than just ordinary carelessness".
In the book, due to be released later this week, Mr Comey compares Mr Trump to a mafia don and calls his leadership of the country, "ego driven and about personal loyalty".
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"The question is, how much stain is too much stain and how much stain eventually makes you unable to accomplish your goal of protecting the country and serving the country?" he told anchor George Stephanopoulos.
And the book promises to be a damning look at the Trump White House from the perspective of a man who was sacked by the 45th president.
In the first excerpt of the interview, broadcast on "Good Morning America", Comey said he cautioned Trump against ordering an investigation into a salacious intelligence dossier alleging an 2013 encounter with prostitutes in Moscow.
Asked to address the fierce criticism he received for the move, Mr. Comey said Americans would benefit from stepping into his shoes and considering the options he had. "But I can't, it's possible".
Later in the day, Comey wrote on Twitter that his book, "A Higher Loyalty", is about "ethical leadership". The New York Times published a full-page op-ed reminding the President he's not above the law, a reminder of how real fears of a systemic political crisis in the event Trump attempts to fire special counsel Robert Mueller or Deputy Attorney General Rod Rosenstein, who is overseeing his probe. Trump wrote on Twitter Sunday.
Even so, Comey said his preferred solution to the Trump presidency would not be impeachment, but a rejection by voters.
However he insisted he would have still sent the letter to congress which announced the FBI was re-opening investigations into Mrs Clinton's email, just days before the election, even if he knew the letter would elect Mr Trump.
Mueller is investigating Russian interference in the 2016 presidential election, and possible collusion by Trump campaign officials in that meddling. Previously, Comey's defenders have argued that a Russian intelligence document the Federal Bureau of Investigation received in early 2016 suggesting Lynch would protect Clinton in some fashion from the email probe meant that he had to cut her out of the final decision-making process. "I don't remember spelling it out, but it had to have been, that she's going to be elected president and if I hide this from the American people, she'll be illegitimate the moment she's elected, the moment this comes out".