During almost three hours of blockbuster sworn testimony to a Senate panel, Comey described himself as "stunned" by Trump's "very disturbing" and "very concerning" behavior during several private meetings.
Expanding on the bombshell statement released on the eve of his appearance, Comey said the president asked him for "loyalty" during a White House dinner and to lay off former national security advisor Mike Flynn -- who is under criminal investigation -- imploring Comey to "let this go."
"It's my judgment that I was fired because of the Russia investigation," he told senators. "I was fired in some way to change, or the endeavor was to change the way the Russia investigation was being conducted. That is a very big deal."
Detailing private talks with a sitting president, which under normal circumstances would never see the light of day, Comey said he took painstaking notes of the extraordinary encounters for fear Trump might "lie" about the meetings.
Comey indicated that it was now up to a high-powered special prosecutor to determine whether the president's behavior constituted an obstruction of justice, a potentially impeachable offence.
Trump avoided directly responding to the explosive accusations, defiantly telling supporters at a religious event in the capital: "We are going to fight and win."
But the White House hit back angrily at Comey.
"I can definitely say the president is not a liar and frankly am insulted by that question," said White House spokeswoman Sarah Sanders.
Easing months of speculation Comey did confirm that Trump was not personally the subject of a counterterror or criminal probe when he left the FBI last month.
The probe into Russian meddling in the 2016 election has ensnared close aides of the president and has vast-ranging political and geopolitical implications.
Watch: Comey concerned Trump 'may lie'
'Lies, plain and simple'
After solemnly raising his right hand and vowing to tell the whole truth, a visibly aggrieved Comey kicked-off his testimony with a bid to set the record straight about the state of the bureau he led until he was sacked last month.
"Although the law requires no reason at all to fire an FBI director, the administration then chose to defame me and more importantly the FBI by saying that the organization was in disarray, that it was poorly led, that the work force had lost confidence in its leader," he charged.
"Those were lies plain and simple," Comey said, firing a shot of tension through hearing room 216 of the Senate's Hart building, which stood silent except for the shutter clicks of a phalanx of photographers, there to capture this moment of pure political theater.
Thursday's hearing started with a call for unity of purpose.
"We are here because a foreign adversary attacked us right here at home. Plain and simple," said Senator Mark Warner, the ranking Senate Intelligence Committee, in opening remarks.
"Not by guns or missiles, but by foreign operatives seeking to hijack our most important democratic process -- our presidential election."
But the pretence of bipartisanship soon fell away.
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Democrats are intent on determining whether Trump's actions amounted to obstruction of justice, while Republicans have zeroed in on Comey's admission he assured the president he was not personally an FBI investigation target.
Republicans also pounced on Comey's admission that he leaked personal notes on his meetings with Trump to prompt the naming of a special prosecutor to lead the Russia probe.
That prompted Trump's son Donald junior to describe Comey sarcastically as a "classy guy."
A private lawyer for Donald Trump denied Comey's bombshell allegations, and suggested the ousted lawman should be prosecuted for leaking "privileged information."
The president, said Marc Kasowitz, "never told Mr. Comey 'I need loyalty, I expect loyalty' in form or substance," rejecting a key allegation made by the sacked FBI director before a Senate panel Thursday.
Rejecting key parts of Comey's damning testimony while claiming others as a win for the president, Kasowitz also suggested Comey may face prosecution.
"Today, Mr. Comey admitted that he leaked to friends his purported memos of these privileged conversations, one of which he testified was classified."
"We will leave it the appropriate authorities to determine whether this leaks should be investigated along with all those others being investigated."
WATCH: Trump's lawyer disputes Comey claims
'Very disturbing'
Trump abruptly fired Comey as director of the Federal Bureau of Investigation on May 9, admitting later that the Russia probe was on his mind at the time.
In his written statement, Comey described his mounting discomfort in the weeks leading up to his dismissal as Trump pulled him aside in one-on-one encounters and in phone calls to press him on the probe into Trump campaign associates and possible collusion with a Russian effort to tilt the 2016 vote in the Republican's favor.
At a private White House dinner on January 27, just days after the billionaire took office, Comey said Trump appeared to want to "create some sort of patronage relationship" with him.
"The president said, 'I need loyalty, I expect loyalty.' I didn't move, speak, or change my facial expression in any way during the awkward silence that followed," Comey said.
In an Oval Office tete-a-tete the following month Comey said Trump pressed him to drop the FBI investigation into Flynn, who had been fired for lying to the vice president about his unreported conversations with the Russian ambassador.
And he described trying to insulate himself and the FBI from political pressure in the weeks before Trump fired him, as the president complained about the Russian probe and labeled it "fake news."
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Trump 'vindicated' by Comey
Networks and cable news stations provided blanket coverage of the hearing, and a number of bars in Washington were opening early, with TVs tuned to live broadcasts of the hearing -- one of them offering free drinks every time Trump tweets about Comey.
At Shaw's Tavern where the menu featured an FBI breakfast of French Toast with maple syrup, hundreds watching fell silent as Comey entered the hearing.
'There's been so much media hype, it's good to actually hear this from the source,' said Sadie Cornelius, 33. "It's good to hear the facts."
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Democrats have been quick to draw parallels with the Watergate scandal, when president Richard Nixon, facing impeachment for obstruction of justice, was forced to resign in 1973.
But the White House and Trump's allies have sought to put a positive spin on Comey's revelations.
"The president feels completely and totally vindicated. He is eager to continue to move forward with his agenda," said Marc Kasowitz, hired by Trump as his personal lawyer to deal with issues linked to the Russia investigation, after Comey's statement was released.
