Earlier, Trump tweeted criticism of Comey, denying some of Comey's allegations and alleging that Comey revealed classified information and lied to Congress.
"Slippery James Comey, a man who always ends up badly and out of whack (he is not smart!), will go down as the WORST FBI Director in history, by far!" Trump wrote.
The Washington Post was allowed to review a complete transcript of the Comey interview, which lasted nearly five hours.
As he did in his book, Comey detailed in the interview Trump's fixation on unproven allegations that he watched prostitutes urinate on one another in a Moscow hotel in 2013, asserting that Trump at one point said he was contemplating ordering Comey to investigate and disprove the incident because he did not want "even a 1 percent chance" that his wife, first lady Melania Trump, would believe it happened.
Comey said that struck him as odd. "I remember thinking, 'How could your wife think there's a 1 per cent chance you were with prostitutes peeing on each other in Moscow?' " he said, adding that his assessment was it's possible Trump is guilty of the accusation.
"I honestly never thought these words would come out of my mouth, but I don't know whether the current president of the United States was with prostitutes peeing on each other in Moscow in 2013," Comey said. "It's possible, but I don't know."
Comey said it was possible, too, that the Russians might have material that could be used to blackmail Trump.
"Do you think the Russians have something on Donald Trump?" Stephanopoulos asked.
"I think it's possible. I don't know. These are more words I never thought I'd utter about a president of the United States, but it's possible," Comey responded.
Comey described in great detail several conversations he had with Trump, telling Stephanopoulos of how the President asked for his loyalty and how that interaction and others reminded him of his time as a prosecutor in New York pursuing mob families, for whom loyalty to the boss and the organisation were the only values that mattered.
"It's the family, the family, the family, the family," Comey said.
Trump has denied asking for Comey's loyalty.
Comey offered a blunt assessment of a conversation with Trump on February 14, 2017, during which Comey maintains the President said of an investigation the FBI was conducting into former national security adviser Michael Flynn, "I hope you can let it go." Trump disputes Comey's account.
"With that direction, was President Trump obstructing justice?" Stephanopoulos asked.
"Possibly," Comey responded. "I mean, it's certainly some evidence of obstruction of justice. That something really important just happened and that I was a little - another one of those outta-body experiences, like, 'Really? The president just kicked out the attorney-general to ask me to drop a criminal investigation.' Wow, the world continues to go crazy."
Comey even took aim at Trump's personal appearance, remarking how his "tie was too long, as it always is" and that his face "looked slightly orange up close with small white - half moons under his eyes, which I assume are from tanning goggles."
The former FBI boss acknowledged he had grave misgivings about the Trump presidency even before it began.
In a meeting with President Barack Obama in the last days of his Administration, Comey says he told the President: "I dread the next four years. But in many ways, I feel great pressure to stay to try and protect the institution I lead."
While Trump bore the brunt of his criticism, Comey also took aim at others - including Deputy Attorney-General Rod Rosenstein, whom Trump has been contemplating removing from his post.
Comey said Rosenstein had "acted dishonorably" in authoring a memo lambasting Comey's handling of the investigation into Hillary Clinton's use of a private email server while she was secretary of state. Trump cited the memo in firing Comey, and Comey said he came to believe Rosenstein was "part of the family now. I can't trust him."
He later said, though, that he did not believe Rosenstein would fire Special Counsel Robert Mueller if ordered by Trump to do so, and that Rosenstein "has an opportunity in overseeing Bob Mueller to restore some of his professional reputation."
As he has in the past, Comey offered a vigorous defence of his handling of the Clinton email investigation.
Parts of the interview are likely to revive the fury of Clinton supporters who think he cost her the presidency by reopening the email investigation less than two weeks before the election.
When Stephanopoulos asked him if the decision was "influenced by your assumption that Hillary Clinton was going to win," Comey replied: "It must have been. I don't remember consciously thinking about that, but it must have been. 'Cause I was operating in a world where Hillary Clinton was going to beat Donald Trump."
He also said he was sorry for how he handled the first announcement in July 2016 that he was closing the Clinton email probe without seeking any charges. He says he agrees now with the criticism that his remarks muddied important issues.
"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.
He also spoke at length about his complicated relationship with former Attorney-General Loretta Lynch. Previously, Comey's defenders have argued that a Russian intelligence document the FBI 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.
But the Washington Post has reported that many FBI officials viewed the allegation against Lynch as dubious at best - and possibly one of Russia's very first disinformation efforts during the 2016 campaign.
Comey said he didn't believe the allegation, but feared that if it ever came out, it would destroy the credibility of the Justice Department and the FBI.
"There was material that I knew someday, when it's declassified, and I thought that would be decades in the future, would cause historians to wonder, 'Hmm, was there some strange business goin' on there? Was Loretta Lynch somehow carrying water for the campaign and controlling what the FBI did?' "
So partly for that reason, he said, Comey decided to announce on his own his recommendation that no charges be filed in the Clinton case.
Throughout the interview, Comey stressed the importance of telling the truth, a theme in his book. He described being initially reluctant to prosecute Martha Stewart for lying to investigators, but then recalled a case when he was a federal prosecutor in Richmond and had charged a minister with the same thing.
"And there once was a day when people were afraid of goin' to hell if they took an oath in the name of God and violated it. We've drifted away from that day. And so in its place has to be a fear that if you lie and the government can prove it beyond a reasonable doubt, they will prosecute you in order to send a message to all the others who might be called upon to give evidence," Comey said. "We must prosecute people who lie in the middle of an investigation."
The comments come a day after the Justice Department inspector general released a report accusing Andrew McCabe, Comey's former deputy, of lying repeatedly as they investigated a media disclosure he had authorised. The inspector general says McCabe even lied to Comey, though McCabe disputes Comey's account.
After he was fired, Comey said, Trump issued an order that he was not to be allowed back in the FBI building, even to retrieve his belongings. His firing came as Comey was visiting the FBI office in Los Angeles, and for a brief moment it wasn't even clear if he would be allowed to ride on the government plane back to Washington.
When he did get on the plane, he decided to open a bottle of wine. "I drank red wine from a paper cup and just looked out at the lights of the country I love so much as we flew home," he said.
