Donald Trump's lawyer is denying there was any pressure on the former FBI director James Comey to drop an investigation.
Mr Comey told senior US politicians the president's decision to sack him was an effort to obstruct a probe into Russian activities during the election.
But Mr Trump's attorney, Marc Kasowitz, has suggested Mr Comey may face prosecution for leaking some information.
"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", he said.
The former FBI director said the shifting explanations for his firing by the Trump administration "confused me and concerned me".
US Senators questioned James Comey earlier Thursday - a month after he was fired by Donald Trump.
Mr Comey appeared before the Senate Intelligence Committee as part of their ongoing investigation into alleged Russian interference in last year’s presidential election.
Questioned over his claim that President Trump asked him to 'let go' of the investigation into former national security adviser Michael Flynn, Mr Comey added: "I don't think it's for me to say whether the conversation I had with the President was an effort to obstruct".
On the subject of his decision to document his first meeting with the then president-elect, Mr Comey observed: "I was honestly concerned he might lie about the nature of our meeting, so I found it really important to document."
He also said that he believes the Russia investigation was a reason for his firing "because I've seen the president say so".
When asked if Trump had colluded with the Russians, he said he could not answer in a open setting.
Mr Comey confirmed that he asked a friend to share details of a memo - detailing one of his meetings with Mr Trump - with a reporter so that it "might prompt appointment of a special counsel".
"Russians interfered in our election"
He also told senators that the FBI was aware of details that would have made Attorney General Jeff Sessions' "continued engagement in a Russia investigation problematic". Mr Sessions has previously announced he will recuse himself from any Russia probe.
On Russia's efforts during the presidential campaign, Mr Comey insisted: "There should be no fuzz on this whatsoever - the Russians interfered in our election [...] They did it with purpose, they did it with sophistication."
Later, he observed: "We're talking about a foreign government that, using technical intrusion and other methods, tried to shape the way we think, we vote, we act - that is a big deal."
He also suggested he believes it is part of an ongoing effort, noting: "They'll be back".
Donald Trump has refused to respond to explosive allegations by the FBI's former director.
Without referring to that testimony directly, Mr Trump vowed to fight his enemies and knock back attempts to derail him.
"We're under siege, you understand that - but we will come out bigger and better and stronger than ever, you watch", he said.
A written opening statement prepared by Comey was published ahead of today's hearing.
In the statement, the former FBI head details his one-on-one meetings and conversations with President Trump.
Backing up Trump’s own assertions, Comey says he assured the president three times that he was not personally under investigation as part of the Russia probe.
Comey does, however, note that he was ‘concerned’ about a number of his interactions with the president - including over an apparent request that the FBI drop an investigation into Michael Flynn.
In an apparent confirmation of a New York Times report from last month, the statement claims Trump said: “I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go.”
Comey also alleges that Trump told him he ‘expects loyalty’.
According to the testimony, Comey - who was appointed as FBI director in 2013 - felt ‘compelled’ to document his meetings with Trump.
He says: “This had not been my practice in the past. I spoke alone with President Obama twice in person (and never on the phone) - once in 2015 to discuss law enforcement policy issues and a second time, briefly, for him to say goodbye in late 2016.
“In neither of those circumstances did I memorialize the discussions. I can recall nine one-on-one conversations with President Trump in four months – three in person and six on the phone,” he adds.
Senators' questions today will likely attempt to establish whether or not President Trump’s actions constitute an attempt to obstruct justice - a suggestion legal commentators have so far offered mixed opinions on.
Yesterday, NSA Director Mike Rogers and Director of National Intelligence Dan Coats refused to be drawn on whether or not they’d been asked to intervene in the Russia investigation.
In his only statement on the subject, Rogers said: "In the three-plus years that I have been director of the National Security Agency, I have never been directed to do anything I believe to be illegal, immoral, unethical or inappropriate - and, to the best of my recollection, during that same period of service I do not recall ever feeling pressured to do so.”