Donald Trump's ex-lawyer has claimed Mr Trump wanted him to lie about a property deal in Moscow amid his election campaign.
In a blistering opening statement, Cohen branded Mr Trump a "racist", a "conman" and a "cheat".
Michael Cohen's allegations:
- Michael Cohen assailed the president's character calling him a racist, a conman and a cheat
- Cohen also said Mr Trump directed plans for a Moscow skyscraper even during his White House campaign.
- He said Mr Trump had not released his tax returns because he feared that public scrutiny would lead to an audit and tax penalties. As a presidential candidate in 2016, Mr Trump broke with a decades old practice of making his tax returns public and continues to refuse to release them as president.
- Cohen testified Mr Trump asked him to pay off porn star Stormy Daniels, who alleges she had had an affair with Mr Trump, and to lie to Melania Trump about it.
- Cohen said for the first time that Mr Trump knew in advance about a WikiLeaks dump of stolen emails that hurt his 2016 election rival Hillary Clinton.
- He said he feared that if Mr Trump loses the the election in 2020 "there will never be a peaceful transition of power."
- House Oversight Committee chairman Eligjah Cummings said it appeared Mr Trump had committed a crime but declined to say whether he should be removed from office.
Cohen, 52, will start a three-year prison term in May for the campaign finance violation of paying hush money to one of Mr Trump's alleged mistresses, tax evasion and lying to Congress.
In his public testimony to the House of Representatives Oversight Committee on Wednesday, he said Mr Trump "knew of and directed" plans for a Trump Tower Moscow, while stating publicly that he had no dealings in Russia.
"At the same time I was actively negotiating in Russia for him," Cohen testified, "he would look me in the eye and tell me there's no business in Russia and then go out and lie to the American people by saying the same thing. In his way, he was telling me to lie."
"He wanted me to lie," the witness added.
However, Cohen has been convicted of lying to Congress when he testified in 2017 that attempts to build a Trump skyscraper in Moscow had stopped by January 2016.
He has since acknowledged negotiations actually continued until June 2016 in the midst of the election campaign, though the real estate project ultimately did not go ahead.
Cohen apologised on Wednesday for his earlier false statements to Congress, which he claimed were "reviewed and edited" by Mr Trump's lawyers.
Jay Sekulow, counsel to President Trump, said in a statement after the hearing: "Today's testimony by Michael Cohen that attorneys for the president edited or changed his statement to Congress to alter the duration of the Trump Tower Moscow negotiations is completely false."
Cohen also suggested federal prosecutors in New York are investigating some unspecified crime involving Trump.
Cohen said he was in Mr Trump's office in July 2016 when Roger Stone, a longtime political adviser, called the then-Republican presidential candidate.
The witness said Mr Stone rang Mr Trump to let him know he had been speaking to Wikileaks founder Julian Assange, who told him there would be a "massive dump" of emails within a couple of days that would politically embarrass Hillary Clinton's White House campaign.
Cohen said Mr Trump responded along the lines of "wouldn't that be great".
Mr Trump has denied having prior knowledge about Wikileaks' disclosure of Democratic National Committee (DNC) emails during the election.
The messages - which US authorities say were hacked by Russian intelligence - caused a damaging rift among Democrats by exposing how party officials preferred Mrs Clinton over her challenger for the presidential nomination, Bernie Sanders.
Mr Stone, a self-proclaimed political dirty trickster, is currently facing charges of lying to Congress about his communications with Wikileaks and witness tampering.
Trump Tower meeting
Cohen also testified that, contrary to Mr Trump's repeated claims, he did have advance knowledge of a meeting at Trump Tower in Manhattan between his campaign aides and a Russian lawyer promising "dirt" on Mrs Clinton.
The June 2016 meeting has been investigated by special counsel Robert Mueller, who is winding up a 21-month justice department inquiry into whether the Trump campaign colluded with an alleged Kremlin plot to influence the 2016 US presidential election.
Cohen spoke about an incident when Mr Trump's son, Donald Jr, walked behind his father's desk and told him in a low voice: "The meeting is all set."
Mr Trump, Cohen told the hearing, replied: "OK good, let me know."
Importantly, Cohen also said under oath that he has no direct evidence that Mr Trump or his campaign colluded with Russia.
"I do not," he said. "I want to be clear. But I have my suspicions."
Mr Cohen told lawmakers Mr Trump is a racist.
He said: "He once asked me if I could name a country run by a black person that wasn't a 'shithole.'
"This was when Barack Obama was President of the United States.
"While we were once driving through a struggling neighbourhood in Chicago, he commented that only black people could live that way.
"And he told me that black people would never vote for him because they were too stupid. And yet I continued to work for him."
Cohen provided what he said was evidence of reimbursements he received from the president for hush money the lawyer has admitted paying to a porn star who says she had an affair with Mr Trump.
He submitted to the committee a copy of his $130,000 (£97,000) wire transfer to Stormy Daniels to keep her quiet - a payment which led to Cohen's conviction for campaign finance violations.
Cohen also gave the panel a copy of a $35,000 cheque dated August 2017 - one of a series he said Mr Trump signed to pay him back in instalments.
"Lying to the First Lady is one of my biggest regrets," Cohen said. "She is a kind, good person. I respect her greatly - and she did not deserve that."
The 52-year-old was convicted last year by New York federal prosecutors of campaign finance violations and tax evasion and by special counsel Robert Mueller of lying to Congress about Trump Organization plans in Moscow.
Committee members' response
During the hearing, the president's fellow Republicans pilloried committee Democrats for inviting a man convicted of lying to Congress.
Jim Jordan of Ohio called Cohen - who lost his law licence on Tuesday - a "fraudster" and "cheat".
But committee chairman Elijah Cummings, a Maryland Democrat, defended the decision to give Cohen a public platform, saying it was the panel's job to search out the truth.
After the hearing, when asked whether the president committed a crime while in office, Chairman Cummings said: "It appears that he did".
He did not offer any further detail on what crime he thought Mr Trump committed.