(AFP) - Ousted FBI chief James Comey accused Donald Trump's White House of lies and defamation Thursday in wounding sworn testimony that plunged the US leader's already troubled presidency deeper into peril.

During almost three hours of extraordinarily frank statements carried live by networks and cable news stations, Comey described himself as "stunned" by Trump's "very disturbing" and "very concerning" behavior in several meetings.

Detailing one-on-one talks with a sitting president -- which under normal circumstances are private -- Comey said he took painstaking notes for fear Trump might "lie" about the unusual encounters.

The testimony painted a devastating picture of an untrustworthy president, who at best unknowingly shred the norms of office by pressing Comey on the probe into Russian election meddling, and at worst may have criminally obstructed justice.

During one White House dinner, Comey recalled that the president asked him for "loyalty" and to lay off his former national security advisor Michael Flynn -- who is under criminal investigation over his Russia ties -- imploring Comey to "let this go."

Comey indicated it was now up to a high-powered special prosecutor to determine whether that behavior, and his own sacking, constituted an obstruction of justice, a potentially impeachable offense.

"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."

Easing months of speculation, Comey confirmed that Trump was not personally under investigation when he left the FBI last month.

The White House and Trump's lawyers expressed vindication over some parts of Comey's testimony and lashed out at others.

"I can definitely say the president is not a liar and frankly am insulted by that question," White House spokeswoman Sarah Huckabee Sanders said.

Trump's lawyer Marc Kasowitz said the president "never told Mr Comey 'I need loyalty, I expect loyalty' in form or substance," rejecting a key allegation.

Deploying Trump's trademark bareknuckle style, Kasowitz also suggested the ousted lawman should be prosecuted for leaking "privileged information."

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."

While the president was unusually quiet on his preferred communication platform, Twitter, his son Donald Trump Jr live-tweeted throughout Comey's testimony.

"You see how disgusting the swamp really is, the people that are there," the younger Trump said in an interview with Fox News later Thursday, a nod to his father's campaign pledge to "drain the swamp" that is Washington.

"This is what the people voted against last time. I'm happy for them to keep doing it. It's going to make 2020 that much easier," he said, referring to the next presidential election.

- 'Lies, plain and simple' -

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 photographers capturing the political theater.

Democrats are intent on determining whether Trump's actions amounted to obstruction of justice, while Republicans zeroed in on Comey's admission he assured the president he was not personally an FBI investigation target.

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 a written statement, Comey described his mounting discomfort in the weeks leading up to his dismissal as Trump pulled him aside in person and phoned to press him on the probe into his campaign associates and possible collusion with a Russian effort to tilt the 2016 vote in the Republican's favor.

- 'A big deal' -

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 unreported conversations with the Russian ambassador.

Harvard Law School professor Mark Tushnet said onlookers should keep in mind that Comey knows much more than he can say.

"He's saying that when he looks at all the evidence that he knows, that he draws the conclusion that the president was trying to pull him off the investigation of Flynn."

"It's a pretty powerful statement that an investigator with his background (says) when I look at this investigation, this is what I conclude. So his statement is a big deal."

As the hearing played out, Republican House Speaker Paul Ryan appeared to sympathize with Trump, telling a news conference that "the president's new at this."

"He probably wasn't steeped in the long-running protocols that establish the relationships between (Department of Justice), FBI and White Houses. He's just new to this."

© Agence France-Presse