"I hardly know the man [Mr Comey]", he said.
The key issue before the committee was whether Mr Trump sought to obstruct the investigation in nine conversations with Mr Comey this year, leading up to his dismissal of the FBI chief.
Mr Trump's private lawyer, Marc Kasowitz, seized on the admission, casting the former Federal Bureau of Investigation director as one of the "leakers" set on undermining the Trump administration.
Comey said he believed he was sacked because of the Russian Federation probe, which has moved steadily closer to the White House, with investigators examining Trump's son-in-law and senior adviser Jared Kushner.
The former FBI director also appeared to confirm Trump's statements that, on three occasions, Comey told the president he was not personally under investigation with regard to Russian Federation.
Mr Comey's testimony is certain to dominate the question-and-answer session when Mr Trump faces the press at the White House later on Friday in a joint news conference with Romania's president. And, "You're going to be very disappointed when you hear the answer".
Comey said he told Trump in multiple conversations that he was not being personally investigated and said the president implored him to make that public.
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.
Trump also said that Comey's testimony "vindicated" the president and that everything in it was "basically ridiculous".
Trump's offer to testify heightens the drama of his standoff with Comey, who told senators that Trump had demanded his personal loyalty and had pressured him to drop a counterintelligence investigation of former National Security Adviser Michael Flynn.
Top Aides to UK PM Quit After Election Losses
Theresa May also forced her two closest aides to quit on Saturday after they have taken the blame for the election campaign. May said Barwell would help her "reflect on the election and why it did not deliver the result I hoped for".
On Thursday, Comey confirmed to the Senate Intelligence Committee that he did write the memos, which he then turned over to his friend Professor Daniel Richman of Columbia Law School to leak to the press.
". and WOW, Comey is a leaker!"
"And there would be nothing wrong if I did say it according to everybody that I read today, but I did not say that".
Friday's tweet was the first since Thursday's hearing.
"If you're talking about a credibility contest here, I don't think it's going to be close", said lawyer Nick Akerman, an assistant prosecutor in the Watergate case.
During nearly three hours of statements Thursday before the Senate Intelligence Committee, Comey described himself as "stunned" by Trump's "very disturbing" and "very concerning" behavior towards him.
The president had previewed his attacks against Mr Comey in an early-morning tweet that broke his previous day's silence on his favorite social media megaphone.
"We ask Qatar and other nations in the region to do more and do it faster", Trump said. The White House has denied collusion with Moscow. The president also said the investigation is an "excuse" by the Democrats before they lost the election.
A few days later, Trump Jr. linked to a news article posted on the far-right news site Breitbart that suggested Muslim men are "a menace" to Western women and posted a doctored image of himself, his father and several prominent Trump allies next to Pepe the Frog, a cartoon character whose image has been used by white supremacists.
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