Ex-FBI chief James Comey told Congress that the Trump administration's comments about him and the FBI were "lies plain and simple".
Mr Comey told a Senate committee they were wrong to denigrate the agency and its leadership.
He was also "confused" by the "shifting explanations" for his sacking, which came as he led a probe into any links between the Trump campaign and Moscow.
Mr Comey said Mr Trump had repeatedly told him he was doing a "great" job.
But he also suggested he was fired to "change the way the Russia investigation was being conducted".
"I know I was fired because something about the way I was conducting the Russia investigation was some way putting pressure on him, was some way irritating him," Mr Comey said of the president.
The former FBI boss remained largely composed throughout almost three hours of testimony, but became impassioned when delivering his opening remarks.
He told the panel that the White House "chose to defame me, and more importantly the FBI" by claiming the agency was "poorly led".
"Those were lies, plain and simple. And I'm so sorry that the FBI workforce had to hear them," he continued.
Mr Comey was leading one of several Russia investigations before Mr Trump fired him.
US intelligence agencies believe Russia interfered in the US election and they are investigating alleged links between the Trump campaign and Moscow.
But there is no known evidence of collusion and President Donald Trump has dismissed the story as "fake news".