The hours-long Capitol Hill grilling comes amid turbulence at the White House, and mounting intrigue over Special Counsel Robert Mueller's investigation into Russian interference in the 2016 election and possible contacts between Trump's presidential campaign and Moscow.
Comey smiled as he walked past reporters towards a closed-door House meeting, telling them "maybe later" he would answer questions.
Trump's bete noire had pleaded for a public hearing after he was subpoenaed by members of the outgoing Congress in November, but House Republicans including some of Trump's allies insisted on a private session before the judiciary and oversight committees.
Comey was questioned as part of a Republican-led House inquiry into possible Russian interference in the US election, and email use by Democrat Hillary Clinton, who lost to Trump in 2016.
Congressman Darrell Issa said he wanted answers about what he called the "fake dossier," a 2016 intelligence report including opposition research that alleges misconduct and ties between the Trump campaign and Russia.
"We want to know what he knew and when he knew it," Issa said of Comey. He also complained that Comey's lawyers were preventing the former FBI chief from answering some questions from lawmakers.
Three months earlier the president met privately with Comey and urged him to end the investigation into former national security advisor Michael Flynn, a move that many Democrats interpreted as an obstruction of justice.
Flynn, who was indicted for lying to investigators, has been cooperating with Mueller's probe. Mueller was expected to provide court filings Friday related to Trump's jailed former campaign chairman Paul Manafort and former lawyer and longtime fixer Michael Cohen.
The president has repeatedly blasted Mueller's probe as a "witch hunt," and on Friday unleashed a Twitter tirade against Mueller, Comey and other current and former officials tied to the Russia probe.
"Robert Mueller and Leakin' Lyin' James Comey are best friends, just one of many Mueller Conflicts of Interest," Trump tweeted. Comey's testimony will likely be one of the last sessions conducted by the judiciary and oversight panels this year. Control of Congress shifts in January to Democrats, who are keen to end or alter the probe.
"The entire purpose of this investigation is to cast aspersion on the real investigation, which is Mueller," Nadler told reporters outside Comey's testimony.
Comey had resisted answering questions privately, but struck a deal with Republicans that will see a transcript of the testimony published 24 hours after his interview.
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