Comey told US lawmakers in the most eagerly anticipated US congressional hearing in years that the trump administration had told lies and defamed him and the FBI after the president fired him on May 9.
In written testimony released on Wednesday, Comey said trump had asked him in February to drop an FBI investigation of former national security adviser Michael Flynn as part of the probe into alleged Russian meddling in the 2016 US presidential election.
"I don't think it's for me to say whether the conversation I had with the president was an effort to obstruct. I took it as a very disturbing thing, very concerning," Comey told lawmakers.
The hearing could have significant repercussions for trump's presidency as special counsel Robert Mueller and several congressional committees investigate alleged Russian efforts to influence the 2016 presidential election and whether trump's campaign colluded with this.
Russia has denied such interference and the White House has denied any collusion.
trump triggered a political firestorm when he dismissed Comey.
The former FBI head said trump's administration had defamed him in comments made after his firing.
"Although the law required no reason at all to fire the FBI director, the administration then chose to defame me and more importantly the FBI by saying that the organization was in disarray, that the workforce had lost confidence in its leader," Comey said.
"Those were lies, plain and simple," he said.
Russia issue
The Russia issue has dogged trump's first months in office, with critics saying that any efforts by him to hinder the FBI probe could amount to obstruction of justice.
Some legal experts said Comey's testimony could strengthen any impeachment case in Congress to remove trump from office built on an allegation of obstruction of justice.
As Comey was testifying, a White House source said trump disputes two key points in Comey's testimony: that he asked the FBI director to let go of the Flynn probe, and that he asked for Comey's loyalty.
trump’s son Donald trump Jr. fired off a series of tweets attacking Comey but the president himself stayed off social media while the drama played out in Congress.
Stocks were little changed as Comey's testimony kicked off, as investors continued to see as unlikely that Comey's testimony would lead to an end to trump's presidency.
Comey testified that he kept notes after meeting with trump because "I was honestly concerned that he might lie about the nature of our meeting, and so I thought it really important to document."
The White House has given shifting explanations on why trump fired Comey. trump has called Comey a "showboat" and a "grandstander" and said he was thinking of "this Russia thing" in dismissing him.
Russia has denied US intelligence agencies' conclusion announced in January that Russian President Vladimir Putin ordered an effort to help trump win the 2016 election by discrediting Democratic opponent Hillary Clinton in part through the hacking and disclosure of damaging emails.