WASHINGTON — Former FBI Director James Comey flatly challenged President Trump's honesty Thursday, telling a bipartisan Senate panel — and a national audience — that the president lied about his firing, and conveying deep concern that Trump tried to influence a critical investigation into Russian interference in the 2016 election.

Under oath and bright lights, in a packed hearing room brimming with cameras and anticipation, the longtime prosecutor delivered an unusually blunt assessment of a sitting president. Comey said he took copious notes after their meetings because he worried that Trump would lie about their interactions, might have trampled traditions meant to keep law enforcement free from political interference, and fabricated the reasons for firing him by saying the FBI was in disarray.

"Those were lies, plain and simple," Comey told the Senate intelligence committee.

Instead, Comey said, he believes he was dismissed at least in part to ease the political pressure stemming from the FBI inquiry into Russian meddling and whether any Trump allies took part in it.

And in testimony that for Democrats raised questions about potential obstruction of justice by the president, Comey said he felt pressured to drop a probe into former National Security Adviser Michael Flynn, who met with Russian officials and then lied about it.

Comey called the president's actions "disturbing" but would not say if he believed they constituted illegally hindrance of an investigation. That, he said, would be up to special counsel Robert Mueller to determine.

Speaking to reporters hours after the Senate hearing, Trump's personal attorney, Marc Kasowitz, challenged several of Comey's statements and declared that the testimony had "vindicated" the president. In particular, he noted that the former FBI director confirmed that Trump was not, at least under Comey's watch, personally under investigation.

Republicans on the panel also pointed out that Comey said Trump did not attempt to derail the wider investigation into Russian meddling.

The proceeding marked a critical moment in the months-long swirl of questions surrounding Russia's interference in the election, and came amid high-stakes reviews by the Senate, the House, and Mueller.

While many of the key elements discussed had previously emerged in news reports, it marked the first time since Comey's May 9 firing that he spoke publicly about his interactions with Trump. Republicans on the panel downplayed Comey's suggestions that he was improperly pressured, but did not challenge or attack his account.

In explaining his firing, Comey cited Trump's public and private fuming about the Russia investigation and the damage the story was doing to his nascent administration.

"The endeavor was to change the way the Russia investigation was being conducted," Comey said. "That is a very big deal and not just because it involves me. The nature of the FBI and the nature of its work requires that it not be the subject of political consideration, and on top of that you have the Russia investigation itself is vital, because of the threat."

Comey also admitted that he had a friend leak one of his Trump meeting memos to the New York Times. He said he did it in the hope that the revelation would prompt the appointment of a special counsel — as it did.

The former FBI director said he took that action in response to a Trump tweet saying Comey had better hope there were no recordings of their conversations, a remark some took as a signal that the president surreptitiously taped the meetings.

"Lordy, I hope there are tapes," Comey told the senators. He said he had memorialized his meetings and phone calls with Trump — something he did not do with Presidents George W. Bush or Barack Obama — because "I was honestly concerned he might lie about the nature of our meeting."

Hyped as a historic showdown, the nearly three-hour hearing was aired live by national television networks, streamed broadly, and believed to be watched by millions from desktops, smartphones, and bar stools. Whether Trump was among the viewers was unclear.

Throughout the proceeding, the combative president's Twitter account was dormant. As Comey's testimony neared its end, Trump was giving a speech to an evangelical Christian group. He left it to his private attorney to deliver the most forceful reaction.

"It is now established that the president was not being investigated for colluding with or attempting to obstruct any investigation. … These important facts for the country to know are virtually the only facts that have not been leaked," Kasowitz told reporters. "The president feels completely vindicated and is eager to continue moving forward with his agenda, with the business of this country, and with this public cloud removed."

Kasowitz also disputed several of Comey's statements that contradicted the president, saying Trump had not asked Comey to back off of the Flynn inquiry and had not sought pledges of loyalty from the FBI director.

White House spokeswoman Sarah Huckabee Sanders told reporters, "I can definitively say the president is not a liar."

Comey said his recollections are backed by memos he wrote after each of his nine sit-downs or phone calls with the president.

He testified that he believed Trump tried to gain leverage by asking him about his job in a private White House dinner shortly after Trump took office. "My common sense told me, 'What's going on here is, he's looking to get something in exchange for granting my request to stay in the job.'"

Later, he said, the pressure felt more pointed when, in an Oval Office meeting, the president brought up the Flynn investigation and said he hoped Comey could "let this go."

"I took it as a direction. This is the president of the United States, with me alone, saying, 'I hope this.' I took it as, 'This is what he wants me to do,'" Comey said.

He added, as a former prosecutor, that it was "a significant fact" that Trump had kicked out everyone else in the room, including Vice President Pence and Attorney General Jeff Sessions, before that discussion.

A grim-faced Comey, originally from North Jersey, also touched on what he felt was improper influence last year from Obama's attorney general, Loretta Lynch.

He said he felt "queasy" when Lynch urged him to describe the FBI inquiry into Hillary Clinton's private email server as a  "matter" instead of "an investigation," mirroring language used by Clinton's presidential campaign.

Comey declined to answer many questions, saying he could not discuss them in a public forum — though he met with senators in a private session after the hearing. Among the more tantalizing tidbits was a reference to information Comey said he could not have shared with Sessions, his former boss, while he was still in the job — for reasons he said he could not publicly explain in an open hearing.

Sen. James Risch (R., Idaho) noted that Trump did not order Comey to drop the Flynn inquiry.

"He did not direct you to let it go?" Risch asked.

"Not not in his words, no," Comey said.

Trump's son Donald Jr. wrote on Twitter, "Knowing my father for 39 years when he 'orders or tells'  you to do something there is no ambiguity, you will know exactly what he means."

Trump has accused Democrats of drumming up the Russia issue out of frustration over losing the presidential race. But Comey and senators from both parties cast the issue as a critical investigation into a foreign country's attempts to undermine a U.S. election and American credibility.

"It's not a close call. That happened," Comey said. "That's about as unfake as you can possibly get. It is very, very serious."