Sitting alone at a small table facing a bank of senators who fired question after question, Comey gave short, deliberative answers.
Mr Comey also confirmed that he had shared a memo detailing a meeting with Mr Trump to the press through a friend but did so in the hope that a special counsel might be appointed. But he left aside scathing testimony by Comey, who was sacked in early May, that Trump had tried to derail a probe into his onetime national security advisor - at best, a political miscalculation by a tradition-shredding president, and at worst, a criminal obstruction of justice.
Attorney General Jeff Sessions has agreed to appear before the Senate Intelligence committee.
In his first response to Comey's blunt remarks, Trump offered a window into how the White House may try to contain the controversy that has dogged his administration for months - accusations that his campaign team colluded with Moscow to tilt the 2016 election in his favor. If Trump's intent was to punish Comey for not reshaping the Russian Federation inquiry, that would add to the evidence of wrongdoing. He crossed the line, the Department of Justice noted before Trump fired Comey, in announcing he would reopen the Clinton investigation before the election.
The House intelligence committee sent a letter Friday asking White House counsel Don McGahn whether any tape recordings or memos of Comey's conversations with the president exist now or had existed in the past.
Comey said he didn't believe he was being ordered to drop the probe, but he did take Trump's comments "as a direction" that was meant to be obeyed.
The testimony raised questions about whether Trump sought to obstruct justice, and strongly recalled Comey's similarly dramatic appearance before Congress in 2007, when he detailed his refusal to re-authorize a wiretapping program in defiance of the George W. Bush White House.
With Republicans controlling the US Congress, any move to impeach the president would need the support of Republicans.
Sen. Jack Reed of Rhode Island is an ex officio member of the Senate Intelligence committee.
Russian Federation has denied such interference.
Boris Johnson denies plot to topple UK PM
May tried to reassert her shattered authority at the weekend by announcing her new cabinet with no changes among her top team. "Let's get on with the job".
Trump also saluted the United States' relationship with Romania and praised its contribution to the global fight against terror. "We had an investigation open at the time so that gave me a queasy feeling".
Trump's personal lawyer made Comey's secret gambit a central piece of his defense of the president against the fired lawman's testimony.
"We will leave it the appropriate authorities to determine whether this leak should be investigated along with all those others being investigated".
"A lot of our people view this as just a continual re-litigation of an election the Democrats lost", said Dallas Woodhouse, executive director of the North Carolina GOP, who said 800 Republicans came to a party gathering last weekend to hear Kellyanne Conway and Lara Trump speak. The administration's alternative explanations - including that the Federal Bureau of Investigation was in disarray and was being poorly led - were, he said, "lies, plain and simple". However, Vladeck argued that Comey's leak could not fall under that law because "the memo has no pecuniary value".
Harvard Law School professor Mark Tushnet said onlookers should keep in mind that Comey knows much more than he can say, calling his testimony a "big deal".
But Gerhardt said he didn't think Comey's testimony changed the needle one way or the other as to whether obstruction had occurred.
On Thursday, Mr Comey testified to one of several congressional committees that is also looking into the Russian Federation claims. Trump didn't specify why he thought Comey lied and how he was vindicated.
US stocks closed slightly higher as the market reacted little to Comey's testimony, viewing his testimony alone as unlikely to mark the end of Trump's presidency.
Mr. Comey got down to his real mission in claiming that Mr. Trump had ordered him to drop the FBI investigation of former National Security Adviser Michael Flynn. "I hope you can let this go", Trump said, according to Comey's memo.
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 Mr Trump's actions constituted obstruction of justice.
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