"I know what Mr. Trump is," Cohen told the House Oversight and Reform Committee in opening remarks. "He is a racist. He is a conman. He is a cheat."
Cohen compared Trump to a mobster, implying he sought to intimidate him into not testifying in front of Congress.
"Mr. Trump called me a 'rat' for choosing to tell the truth, much like a mobster would do when one of his men decides to cooperate with the government," Cohen said.
Last year, Cohen, the president's former "fixer," pleaded guilty to fraud and campaign finance violations after admitting he paid hush money to Stormy Daniels and playmate Karen McDougal at the direction of Trump.
In his scathing remarks, Cohen expanded upon racist comments he said the president made to him.
"He once asked me if I could name a country run by a black person that wasn't a 's***hole,'" said Cohen. "This was when Barack Obama was president of the United States. While we were once driving through a struggling neighborhood in Chicago, he commented that only black people could live that way."
Toward the end of his opening remarks, Cohen addressed Trump directly, telling him, "I am no longer your 'fixer,' Mr. Trump."
Thousands of miles away in Hanoi, Vietnam, where Trump is meeting with North Korean dictator Kim Jong Un, the president took his own swing at Cohen ahead of his testimony.
"Michael Cohen was one of many lawyers who represented me (unfortunately)," Trump wrote on Twitter. "He had other clients also. He was just disbarred by the State Supreme Court for lying & fraud. He did bad things unrelated to Trump. He is lying in order to reduce his prison time. Using Crooked’s lawyer!"