Update 7.19pm: Donald Trump's personal lawyer says the president "never, in form or substance" directed former FBI director James Comey to stop investigating anyone.
That includes former national security adviser, Michael Flynn.
Marc Kasowitz is responding to Mr Comey's Thursday morning testimony, in which the fired FBI director said President Trump urged him to drop the Flynn case.
Mr Kasowitz says that the president is "entitled to expect loyalty" from those serving the administration.
But he says President Trump never told Mr Comey: "I need loyalty, I expect loyalty," in form or substance, as Mr Comey claimed.
President Trump tasked Mr Kasowitz late last month with responding to matters arising from various probes of Russian interference in the election.
Mr Kasowitz also accused Mr Comey of "unauthorised disclosures" of "privileged communications" he had with the president.
He said there continues "to be those in government who are actively attempting to undermine this administration with selective and illegal leaks of classified information and privileged communications".
He added: "Comey has now admitted that he is one of the leakers."
Mr Comey said in his testimony that he leaked his memos of his conversations with the president to a friend after a tweet by the president suggested he may have taped the conversations.
Mr Kasowitz says President Trump's team will "leave it to the appropriate authorities" to determine whether the leak should be investigated.
Earlier: Former FBI director James Comey has accused the Trump administration 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 Mr Trump, Mr Comey disputed the Trump administration's justification for his firing last month.
Giving evidence before the Senate intelligence committee, he declared that the administration "defamed him and more importantly the FBI" by claiming the bureau was in disorder under his leadership.
In testimony that exposed deep distrust between the president and the veteran lawman, Mr 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 about the nature of our meeting, so I thought it really important to document," Mr 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."
Mr Comey made his comments as the packed hearing got under way, bringing Washington and parts of the country to a halt as all eyes were glued on televisions showing the hearing.
He immediately jumped into the heart of the fraught political controversy around his firing and whether Mr Trump interfered in the bureau's Russia investigation, as he elaborated on written testimony delivered on Wednesday.
In that testimony, he had already disclosed that Mr 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 Russia ties.
Mr Comey also said in his written testimony that Mr 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.
Senate intelligence committee chairman Richard Burr asked Mr Comey the key question about that encounter, saying: "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?"
Mr Comey replied: "I don't think it's for me to say whether the conversation I had with the president was an effort to obstruct.
"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, Mr Comey revealed that after his firing he had tried to spur the appointment of a special counsel by giving one of his memos about Mr Trump to a friend of his to leak to the press.
"My judgment was I need to get that out into the public square," Mr Comey said.
The Republican National Committee and other White House allies worked feverishly to lessen any damage from the hearing, trying to undermine Mr Comey's credibility by issuing press 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 Mr Trump's own lawyer seized on Mr Comey's confirmation, in his written testimony, of Mr Trump's claim that Mr Comey had told him three times the president was not directly under investigation.
Mr Trump himself was expected to dispute Mr Comey's claims that the president demanded loyalty and asked the FBI director to drop the investigation into Mr Flynn, according to a person close to the president's legal team.
The president has not yet publicly denied the specifics of Mr Comey's accounts but has broadly challenged his credibility, tweeting last month that Mr Comey "better hope there are no 'tapes'" of the conversations.
"Lordy, I hope there are tapes," Mr Comey remarked at one point, suggesting such evidence would back up his account over any claims from the president.
But it was a Democrat, Senator Dianne Feinstein, who asked the question that many Republicans have raised in the weeks since Mr Comey's firing as one media leak followed another revealing Mr Comey's claims about Mr Trump's inappropriate interactions with him.
Raising the Oval Office meeting where Mr Comey says Mr Trump asked him to pull back the Flynn probe, Ms Feinstein asked: "Why didn't you stop and say, 'Mr President, this is wrong,'?"
"That's a great question," Mr Comey said. "Maybe if I were stronger I would have. I was so stunned by the conversation I just took it in."
Mr 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 Mr Trump's campaign.
"Yes," Mr Comey said. "Because I've seen the president say so."
The hearing unfolded amid intense political interest, and within a remarkable political context as Mr Comey delivered damaging testimony about the president who fired him, a president who won election only after Mr Comey damaged his opponent, Hillary Clinton, in the final days of the campaign.
Mrs Clinton has blamed Mr Comey's October 28 announcement that he was reopening an email investigation for her defeat.
She has argued she was on course for victory when Mr Comey's move raised fresh doubts about her.
"If the election were on October 27, I would be your president," Mrs Clinton said last month.
Many Democrats blame Mr Comey for Mrs Clinton's loss, leading Mr Trump to apparently believe they would applaud him for firing Mr Comey last month.
The opposite was the case as the firing created an enormous political firestorm that has stalled Mr Trump's legislative agenda on Capitol Hill and taken over Washington.
Under questioning on Thursday, Mr Comey strongly asserted the intelligence community's conclusion that Russia did indeed meddle in the 2016 election.
"There should be no fuzz on this. The Russians interfered," Mr Comey stated firmly.
"That happened. It's about as unfake as you can possibly get."
Mr Trump has begrudgingly accepted the US intelligence assessment that Russia interfered with the election.
But he has also suggested he does not believe it, saying Russia is a "ruse" and calling the investigation into the matter a "witch hunt".