Former FBI director James Comey accused the Trump administration Thursday of spreading "lies, plain and simple" about him and the FBI in the aftermath of his abrupt firing in dramatic testimony that threatened to undermine Donald Trump's presidency.

As he opened his much-anticipated first public telling of his relationship with Trump, Comey disputed the Trump administration's justification for his firing last month, declaring that the administration defamed him and more importantly the FBI by claiming the bureau was in disorder under his leadership.

"Those were lies, plain and simple," Comey told the Senate intelligence committee. "And I am so sorry that the FBI workforce had to hear them and I'm so sorry that the American people were told them."

In testimony that exposed deep distrust between the president and the veteran lawman, Comey described intense discomfort about their one-on-one conversations, saying he decided he immediately needed to document the discussions in memos.

'I was honestly concerned that he might lie.' — James Comey, former FBI director referring to Donald Trump

"I was honestly concerned that he might lie about the nature of our meeting, so I thought it really important to document," Comey said. "I knew there might come a day when I might need a record of what happened not only to defend myself but to protect the FBI."

Comey made his comments as the packed hearing got underway, bringing Washington and parts of the country to a halt as all eyes were glued on televisions showing the hearing. He immediately dove into the heart of the fraught political controversy around his firing and whether Trump interfered in the bureau's Russia investigation, as he elaborated on written testimony delivered Wednesday.

'Lordy, I hope there are tapes.' — James Comey

In that testimony he had already disclosed that Trump demanded his "loyalty" and directly pushed him to "lift the cloud" of investigation by declaring publicly the president was not the target of the FBI probe into his campaign's ties to Russia.

​Comey also said in his written testimony that Trump, in a strange private encounter near the grandfather clock in the Oval Office, pushed him to end his investigation into former national security adviser Michael Flynn.

The Senate intelligence committee chairman, Republican Richard Burr of North Carolina, asked Comey the key question about that encounter: "Do you sense that the president was trying to obstruct justice or just seek a way for Mike Flynn to save face, given he had already been fired?"

Comey: 'The administration then chose to defame me'0:19

"I don't think it's for me to say whether the conversation I had with the president was an effort to obstruct," Comey replied. "I took it as a very disturbing thing, very concerning. But that's a conclusion I'm sure the special counsel will work towards to try and understand what the intention was there and whether that's an offence."

Later, in a startling disclosure, Comey revealed that after his firing he had tried to spur the appointment of a special counsel by giving one of his memos about Trump to a friend of his to leak to the press.

"My judgment was I needed to get that out into the public square, " Comey said.

Comey testifies no doubt Russians interfered in 2016 presidential election0:49

The Republican National Committee and other White House allies worked feverishly to lessen any damage from the hearing, trying to undermine Comey's credibility by issuing news releases and even ads pointing to a past instance where the FBI had had to clean up the director's testimony to Congress. Republicans and Trump's own lawyer seized on Comey's confirmation, in his written testimony, of Trump's claim that Comey had told him three times the president was not directly under investigation.

Trump himself was expected to dispute Comey's claims that the president demanded loyalty and asked the FBI director to drop the investigation into Flynn, according to a person close to the president's legal team who demanded anonymity because of not being authorized to discuss legal strategy. The president has not yet publicly denied the specifics of Comey's accounts but has broadly challenged his credibility, tweeting last month Comey "better hope there are no 'tapes"' of the conversations.

"Lordy, I hope there are tapes," Comey remarked at one point in his testimony Thursday, suggesting such evidence would back up his account over any claims from the president.

Comey: 'I was honestly concerned that he might lie'0:46

But it was a Democrat Senator Dianne Feinstein of California who asked the question that many Republicans have raised in the weeks since Comey's firing as one media leak followed another revealing Comey's claims about Trump's inappropriate interactions with him.

Raising the Oval Office meeting where Comey says Trump asked him to pull back the Flynn probe, Feinstein asked: "Why didn't you stop and say, 'Mr. President, this is wrong,'?"

"That's a great question," Comey said. "Maybe if I were stronger I would have. I was so stunned by the conversation I just took it in."

Comey was also asked if he believed he was fired because of the bureau's investigation into Russia's meddling in the 2016 election as well as Russia's ties with Trump's campaign.

"Yes," Comey said. "Because I've seen the president say so."

Comey: 'Lordy, I hope there are tapes'0:31

The hearing unfolded amid intense political interest, and within a remarkable political context as Comey delivered damaging testimony about the president who fired him, a president who won the election only after Comey damaged his opponent, Hillary Clinton, in the final days of the campaign. Clinton has blamed Comey's Oct. 28 announcement that he was re-opening an investigation into her emails for her defeat.

She's argued she was on track to a victory when Comey's move raised fresh doubts about her. "If the election were on Oct. 27, I would be your president," Clinton said last month.

Many Democrats blame Comey for Clinton's loss, leading Trump to apparently believe they would applaud him for firing Comey last month. The opposite was the case as the firing created an enormous political firestorm that has stalled Trump's legislative agenda on Capitol Hill and taken over Washington.

Under questioning Thursday, Comey strongly asserted the intelligence community's conclusion that Russia did, indeed, meddle in the 2016 election.

Comey: "They're coming after America"0:39

"There should be no fuzz on this. The Russians interfered," Comey stated firmly. "That happened. It's about as unfake as you can possibly get."

Trump has begrudgingly accepted the U.S. intelligence assessment that Russia interfered with the election. But he has also suggested he doesn't believe it, saying Russia is a "ruse" and calling the investigation into the matter a "witch hunt."

Trump's private attorney, Marc Kasowitz, seized on Comey's affirmation that he told Trump he was not personally under investigation.

"Contrary to numerous false press accounts leading up to today's hearing, Mr. Comey has now finally confirmed publicly what he repeatedly told President Trump privately. That is that the president was not under investigation as part of any probe into Russian interference," Kasowitz said in a news conference Thursday afternoon.

TRUMP-RUSSIA/KASOWITZ

U.S. President Donald Trump's personal attorney, Marc Kasowitz, speaks to the media on Thursday after the congressional testimony of former FBI Director James Comey, at the National Press Club in Washington. (Yuri Gripas/Reuters)

Kasowitz also drew attention to Comey's admission about giving a memo to a friend to leak to the press.

"It is overwhelmingly clear that there have been and continue to be those in government who are actively attempting to undermine this administration with selective and illegal leaks of classified information and privileged communications," Kasowitz said. "Mr. Comey has now admitted that he is one of these leakers."

"We will leave it to the appropriate authorities to determine whether these leaks should be investigated," he said.  

Justice Department responds

The U.S. Justice Department also jumped on parts of Comey's testimony. In a statement released Thursday evening, department spokesperson Ian Prior disputed comments the former FBI director made about Attorney General Jeff Sessions.  

Comey told the Senate panel he knew of several reasons why it would have been problematic for Sessions to remain involved with investigations into alleged Russian involvement in the U.S. presidential election. Sessions recused himself from the investigations in March, citing his involvement in Trump's campaign. The attorney general had come under fire for not disclosing that he had met with Russia's ambassador when he was a senator the year before but insisted he had done nothing wrong.  

In his statement, Prior emphasized that Sessions's participation in the Trump campaign was the sole reason he recused himself. 

In addition, Prior said that the attorney general "was not silent" when Comey made a comment at a staff briefing about proper communications with the White House. 

"[Sessions] responded to this comment by saying that the FBI and Department of Justice needed to be careful about following appropriate policies regarding contacts with the White House," Prior's statement said. 

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With files from CBC News and Reuters