"He leaked CLASSIFIED information, for which he should be prosecuted. He lied to Congress under OATH." Comey said the president was also obsessed with the alleged existence of a video in which Russian prostitutes said to be hired by Trump urinated on the bed in a Moscow hotel room.
The allegations left Trump fuming.
"He is a weak and untruthful slime ball who was, as time has proven, a terrible Director of the FBI," Trump added in a second tweet.
"His handling of the Crooked Hillary Clinton case, and the events surrounding it, will go down as one of the worst 'botch jobs' of history. It was my great honor to fire James Comey!" Comey, fired by Trump in May 2017, says the president has no sense of what is right and wrong.
"This president is unethical, and untethered to truth and institutional values," he writes, according to excerpts released by The New York Times.
"His leadership is transactional, ego-driven and about personal loyalty." The US president lives in "a cocoon of alternative reality" that he tried to pull others around him into, Comey adds, according to The Washington Post.
Meetings with Trump gave Comey "flashbacks to my earlier career as a prosecutor against the Mob," he says.