US President Donald Trump has said he's "100%" willing to testify under oath about his interactions with fired FBI director James Comey.
Mr Trump insists that Mr Comey lied in some parts of the testimony he gave yesterday to the Senate intelligence committee.
Mr Comey testified under oath.
The president said that he never asked the former FBI chief for a pledge of loyalty and never told him he hoped the investigation of former national security adviser Michael Flynn would go away.
He said no-one would ask a man he hardly knew to pledge loyalty to him
But Mr Trump's claimed other parts of Mr Comey's testimony represented "total and complete vindication" in the Russian election-meddling political scandal.
The president, who had not posted on his Twitter account yesterday as Mr Comey accused the administration of spreading "lies", struck back with an early morning tweet in which he said: "Wow, Comey is a leaker."
Mr Comey had laid bare on Capitol Hill months of distrust of the president, bluntly asserting that President Trump had fired him for interfering with the probe of Russia's ties to the Trump campaign.
At his first congressional appearance since his abrupt firing last month, Mr Comey also revealed that he had orchestrated the public release of information about his private conversations with the president in an effort to further the investigation.
President Trump's tweet said: "Despite so many false statements and lies, total and complete vindication ... and WOW, Comey is a leaker."
Mr Comey's testimony provided a gripping account of his interactions with President Trump and underscored the discord that had soured their relationship.
He portrayed President Trump as a chief executive dismissive of the FBI's independence and made clear that he interpreted Mr Trump's request to end an investigation into his former national security adviser as an order coming from the president.
Though Republicans worked to discredit Mr Comey and to blunt the impact of his testimony, the ex-director's statement deepened questions about the basis for his May 9 dismissal and about whether President Trump's actions constituted obstruction of justice.
The veteran lawman expressed confidence that could be a matter ripe for investigation by special counsel Robert Mueller, though he declined to offer an opinion on whether it met such a threshold.
President Trump's private lawyer, Marc Kasowitz, seized on Mr Comey's admission that he had told President Trump on multiple occasions that he was not personally under investigation and maintained the testimony made clear that Mr Trump "never, in form or substance, directed or suggested that Mr Comey stop investigating anyone".
Mr Kasowitz also jumped on Mr Comey's revelation that he had released details of his private conversations with the president, casting the former FBI director as one of the "leakers" set on undermining the Trump administration.
Still, there was no doubt the veteran lawman made for a challenging adversary.
"It's my judgment that I was fired because of the Russia investigation," Mr Comey said toward the end of more than two hours of testimony before the Senate intelligence committee.
"I was fired in some way to change, or the endeavour was to change, the way the Russia investigation was being conducted.
"That is a very big deal, and not just because it involves me."
At one point, he practically dared President Trump to release any recordings of their conversations, a prospect the president once alluded to in a tweet.
"Lordy, I hope there are tapes," Mr Comey said, suggesting such evidence would back up his account over the president's.
Thursday's hearing brought Washington and other parts of the country to a standstill as Americans sat glued to their screens.
In his opening statement, Mr Comey sombrely accused the Trump administration of spreading "lies, plain and simple" in the aftermath of his abrupt sacking, declaring that the administration "chose to defame me and, more importantly, the FBI" by claiming the bureau was in disorder.
He then dove into the heart of the fraught political controversy around his firing and whether President Trump interfered in the bureau's Russia investigation, as he elaborated on written testimony released a day earlier.
In that testimony, Mr Comey said that President Trump demanded his "loyalty" and directly pushed him to "lift the cloud" of investigation by declaring publicly the president was not a target of the FBI probe into his campaign's Russia ties.
He said that when President Trump told him he hoped he would terminate an investigation into Mr Flynn, he interpreted that as a directive.
"I mean, this is the president of the United States, with me alone, saying, 'I hope' this," he said.
"I took it as, this is what he wants me to do."
He said that while he found the February exchange in the Oval Office disturbing, "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".
Mr Comey said that after his firing he actually tried to spur the special counsel's appointment by giving a damning memo he had written about a meeting with Trump to a friend to release to the media.
"My judgment was I need to get that out into the public square," he said.
The February meeting was one of several one-on-one encounters that Mr Comey said made him feel such intense discomfort that he felt compelled to document them in memos.
"I was honestly concerned that he might lie about the nature of our meeting, so I thought it really important to document," he 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."
Later it emerged the intelligence committee is asking White House lawyers whether there are any tape recordings or memos of Mr Comey's conversations with Mr Trump.
The committee's chairman, RepbuMike Conaway and Adam Schiff, sent a letter to White House counsel Don McGahn, asking him whether any such tapes or memos exist now, or had existed in the past.
The committee has also sent a letter to Mr Comey, asking for any notes or memoranda in his possession that would describe discussions he had with Mr Trump.
The committee is seeking the materials by June 23.
A friend of Mr Comey's is also being asked to hand over any memos the former FBI chief has given him.
Mr Comey testified that he'd given a memo including details about a conversation with President Donald Trump to a friend, who was later identified as law professor Daniel Richman.
Mr Comey said he'd asked Mr Richman to share the content of the memo with a reporter.
Mr Richman has now been asked to provide the memo to the panel.