Mr. James B.  Comey said Mr. Donald Trump lied to the American public when he said that the F.B.I. was in disarray and that agents had lost confidence in Mr. Comey. Testifying before Senate on Thursday, Comey said of Trump’s claims:  “Those were lies, plain and simple,” Mr. Comey said in brief opening remarks.

Mr. Trump made that claim when he fired Mr. Comey last month. Mr. Comey said he was confused and concerned by Mr. Trump’s changing explanation for why he fired him.

Mr. Comey learned of his firing from the news media. He offered a heartfelt farewell to his former employees.

“I am so sorry I didn’t get to say goodbye to you publicly,” Mr. Comey said.

Sarah Huckabee Sanders, a White House spokeswoman, said that Mr. Trump watched some of the hearing but was also focusing on other matters. Asked to respond to Mr. Comey’s assertion that the president had lied, she said, “I can definitively say the president is not a liar, and frankly I’m insulted by that question.”

Trump says “we know how to fight.”

Mr. Trump, appearing at a meeting of faith leaders and politicians in Washington as Mr. Comey testified across town, railed against the media and his political opponents — and vowed to battle on.

“We know how to fight better than anybody and we never ever give up we are winners and we are going to fight,” he told hundreds of supporters at the annual Faith and Freedom Coalition’s Road to Majority Conference at the Omni Shoreham Hotel.

Mr. Comey acknowledged for the first time that the F.B.I. was scrutinizing Mr. Trump’s actions. He said Mr. Trump’s conduct fell within “the scope of” the F.B.I.’s investigation but that he was not specifically under investigation.

The F.B.I. is investigating whether anyone in Mr. Trump’s campaign colluded with Russian agents to try to influence the outcome of the presidential election.

Mr. Comey said he turned over his memos to the Justice Department special counsel, Mr. Mueller, the first public suggestion that the special counsel would investigate Mr. Trump’s firing of Mr. Comey.

Some in Congress have suggested that Mr. Trump tried to obstruct justice by firing Mr. Comey. Mr. Mueller has the authority to investigate obstruction. That is not a guarantee that Mr. Mueller will investigate the president but is a sign that he is reviewing the memos.

Mr. Comey said he wrote memos on every interaction with the president.

“I turned them over to Bob Mueller’s investigators,” Mr. Comey said.

Mr. Comey said he began taking notes on his meetings with the president because, from his first interaction with him, during the transition period, he thought Mr. Trump might lie about what was said.

He testified that he documented all of his meetings with Mr. Trump because it was so unusual for him to be discussing ongoing investigations, alone, with a sitting president. Mr. Comey had served in senior law enforcement positions under three presidents.

“The combination of factors just wasn’t present with either President Bush or President Obama,” he said.

Mr. Comey acknowledged that he orchestrated the leak that revealed his account of his conversation with Mr. Trump in which the president asked him to drop the investigation into the former national security adviser.

Mr. Comey said he decided to make the conversation public through an intermediary after Mr. Trump said on Twitter that the former F.B.I. director had better hope there were no tapes of their discussions. He said he did so with the explicit hope of triggering the appointment of a special counsel to investigate Russian election interference.

“I woke up in the middle of the night on Monday night, ‘cause it didn’t dawn on me originally that there might be corroboration for our conversation; there might be a tape,” Mr. Comey said, referring to May 15. “And my judgment was I needed to get that out in the public square so I asked a friend of mine to share the content of the memo with a reporter. Didn’t do it myself for a variety of reasons but I asked him to because I thought that might prompt the appointment of a special counsel. So I asked a close friend of mine to do it.”

A story reporting on the memo was published online in The New York Times on May 16, and in the newspaper the next day. The story attributed the information to “two people who read the memo” without naming them.

Questioned by Senator Susan Collins, Republican of Maine, Mr. Comey did not identify the friend by name but said he is a professor at Columbia University. Daniel Richman, a law professor at Columbia, confirmed Thursday that he was the close friend who served as intermediary.

Senator Mark Warner, the top Democrat on the committee, highlighted the more serious allegations from Mr. Comey’s testimony, emphasizing the importance of his appearance to the panel’s investigation of Russian interference in the 2016 election.

“Let me be clear,” he said. “This is not a witch hunt. This is not fake news. It is an effort to protect our country from a new threat that will not go away anytime soon.”

In response to a question from Mr. Burr, Mr. Comey confirmed that Michael T. Flynn, the former national security adviser, was facing a criminal investigation at the time he was fired. That means he was under criminal investigation while serving as the national security adviser, a job that gave him ready access to the president and almost all the secret intelligence possessed by the country’s spy agencies.

The Times reported last month that Mr. Flynn warned the Trump transition he was under investigation in early January, more than two week before the inauguration. Nonetheless, he was allowed to take up the national security post.

White House officials pushed back on The Times’s story, insisting that the Trump transition — and later, the White House — knew nothing of any criminal investigation, and questioning whether there was even an investigation into Mr. Flynn at the time.

Mr. Comey has now provided public confirmation that there was indeed an investigation underway while Mr. Flynn was serving at the White House.

As the hearing unfolded, Speaker Paul D. Ryan offered an explanation of sorts for how the president spoke with Mr. Comey.

“The president’s new at this,” Mr. Ryan said at a news conference. “He’s new to government. And so he probably wasn’t steeped in the long-running protocols that establish the relationships between D.O.J., F.B.I. and White Houses. He’s just new to this.”

Mr. Ryan seemed sympathetic to the position that Mr. Trump found himself in, noting that the president had been told by Mr. Comey that he was not under investigation, yet faced public speculation that he was, in fact, under investigation.

“Of course the president’s frustrated,” Mr. Ryan said, “and I think the American people now know why he was so frustrated.”

—New York Times