Cohen says Trump knew about political consultant Roger Stone's contact with Wikileaks founder Julian Assange.

He recalls being in Trump's office when Stone called.

"Mr Trump put Mr Stone on the speakerphone. Mr Stone told Mr Trump that he had just gotten off the phone with Julian Assange and that Mr Assange told Mr Stone that, within a couple of days, there would be a massive dump of emails that would damage Hillary Clinton’s campaign. Mr Trump responded by stating to the effect of 'wouldn’t that be great'".r

"The country has seen Mr Trump court white supremacists and bigots," Cohen says.

He references Trump's former remarks referring to poorer countries with an expletive term.

"In private, he is even worse. He once asked me if I could name a country run by a black person that wasn’t an [expletive]. This was when Barack Obama was President of the United States."

With disgust in his tone, Cohen says once, while driving through a "struggling neighbourhood" in Chicago, Trump said "only black people could live that way" and that "black people would never vote for him because they were too stupid".

Cohen says Trump ran for office as a marketing opportunity.

"He had no desire or intention to lead this nation – only to market himself and to build his wealth and power. Mr Trump would often say, this campaign was going to be the 'greatest infomercial in political history.'"

"He never expected to win the primary. He never expected to win the general election. The campaign – for him – was always a marketing opportunity."

Cohen states for the record, that when he pleaded guilty to felonies in court "for the benefit of, at the direction of, and in co-ordination with" an Individual #1, that person was President Donald Trump.

He says working for Trump, being around him, "was intoxicating".

"I wound up touting the Trump narrative for over a decade. That was my job. Always stay on message. Always defend. It monopolised my life."

Image source, Getty Images

Cohen explains Trump did not explicitly tell him to lie to Congress - "that's not how he operates".

"In conversations we had during the campaign, at the same time I was actively negotiating in Russia for him, 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."

Cohen has provided the House committee with the following documents that he says prove he is telling the truth:

• A copy of a cheque signed by Trump from his personal account to reimburse Cohen for a hush money payment
• Financial statements from 2011-2013
• Letters written at Trump's direction that threaten the president's high school, colleges, and College Board to not release his grades or SAT scores

"The last time I appeared before Congress, I came to protect Mr Trump," Cohen says. "Today, I’m here to tell the truth about Mr Trump."

Cummings said that Cohen's statements on Russia mean that questions about Trump's connection with the longtime American adversary are now fair game.

He had earlier said that questions about alleged Russian collusion were off the table, due to ongoing federal inquiries.

Cohen has provided evidence of a $131,000 money transfer that he claims was used as a hush money payment for adult film star Stormy Daniels.

Committee Chair Elijah Cummings noted Cohen also has a copy of a cheque Trump signed to reimburse Cohen for this payment.

Cohen says he is ashamed of his own failings and has accepted responsibility by pleading guilty.

"I am ashamed that I chose to take part in concealing Mr Trump’s illicit acts rather than listening to my own conscience. I am ashamed because I know what Mr Trump is," Cohen says.

"He is a racist. He is a conman. He is a cheat."

Image source, AFP/Getty

Cohen begins his testimony by thanking the committee for being sensitive about questions related to ongoing investigations and for protecting his family from "presidential threats".

He says the documents he will present are "irrefutable".

"Never in a million years did I imagine, when I accepted a job in 2007 to work for Donald Trump, that he would one day run for President, launch a campaign on a platform of hate and intolerance, and actually win."

"I regret the day I said 'yes' to Mr Trump. I regret all the help and support I gave him along the way."

Republican Jim Jordan has criticised Democrats for inviting to the committee a man about to report to prison in two months.

"Here we go," he says, alleging that the entire hearing is presented by Cohen's lawyer, Lanny Davis, who he calls a "Clinton operative".

"Certainly it's the first time a convicted perjurer has been brought back to be a star witness in a hearing," he says, calling him a "fraudster" and "a cheat".

"They don't care, they just want to use you, Mr Cohen. You're their patsy today, Mr Cohen."

As has grown common in the Trump era, pubs in Washington DC are opening early and offering drink specials to people viewing the hearing.

Duffy’s Irish Pub plans to show it on "eight large flat screen TVs with stadium surround sound tuned in to the Cohen Show".

They're also selling a "Flipper" cocktail, which the owner says is "Russian influenced, a little bitter and 100 percent pardon free".

Image source, AFP/Getty Images

Oversight Chairman Elijah Cummings said Americans must make their own evaluations of Cohen's credibility.

"As he admits, he has repeatedly lied in the past. I agree with ranking member [Jim] Jordan that this is an important factor we need to weigh. But we must weigh it and we must hear from him."

"Where I disagree fundamentally with the ranking member involves his efforts to prevent the American people from hearing from Mr Cohen," Cummings says.

"Mr Cohen's testimony raises grave questions about the legality of President Donald Trump's conduct and the truthfulness of statements while he was president."

Image source, Getty Images

Part of Cohen’s testimony today will centre on Trump Tower Moscow, a Trump Organization project to construct a 100-storey building in Russia.

Trump directed Cohen to spearhead renewed efforts to construct the tower in 2015 – but plans diminished in 2016, as Trump was gearing up to become the Republican presidential nominee.

Cohen fell afoul of the law for lying to Congress about when exactly those plans ended, eventually admitting negotiations continued through June 2016, after Trump had won the nomination.

So how does this impact Trump?

Cohen is saying his original lies about the tower to Congress were at the behest of the president – which, if true, would be a federal crime.

Read more about the Trump Tower that never was here.

Anthony Zurcher
BBC North America reporter

Michael Cohen is unleashing a series of explosive accusations directed against Donald Trump touching on multiple controversies that have bedevilled the president during his time in office.

He suggests the president had advance knowledge of his son’s June 2016 Trump Tower meeting with Russians and that Wikileaks was poised to release damaging information about Democrats. He says the president personally signed checks reimbursing him for a hush money payment to adult film star Stormy Daniels. He alleges that the president was fully aware of ongoing negotiations for a Trump Tower in Moscow well into the 2016 presidential campaign.

Many of these assertions rely almost exclusively on Cohen's word – the word of a man who has already admitted to lying to Congress and to the federal government on his taxes. What’s more, while his allegations are certainly politically damaging, they aren't incontrovertible evidence of legal misconduct by the president.

If there is a case to be made against Mr Trump, it will have to rely on more than the word of his former lawyer and fixer.

That's not to undersell the blockbuster nature of the day's proceedings, however. The public now has a chance to determine, under the glare of the spotlight, whether Cohen lied in the past to protect the president or is lying now to protect himself.

Or, perhaps, a bit of both.

Image source, Getty Images

Michael Cohen, 52, worked for Trump for over a decade as legal counsel, "fixer" and Trump Organization executive before his fall from grace last year.

Cohen - who once vowed he would take a bullet for Trump - has been closest to the president the longest out of all of the special counsel's targets, except for members of Trump's immediate family.

He was convicted of campaign finance violations for hush money payments made to women alleging affairs with Trump, including $130,000 (£97,000) to adult film star Stormy Daniels.

He also pleaded guilty to lying to Congress at least three times over a Trump Organization project to build a tower in Moscow, while Trump was running for the Republican nomination.

At his sentencing hearing in December, Cohen lashed out at his ex-boss' "dirty deeds".

The BBC takes a deeper look at Cohen's past here.

North Carolina Republican Mark Meadows, who chairs the conservative House Freedom Caucus, has expressed outrage that Democrats invited a "convicted liar" to testify.

In an interview with the BBC before the hearing began, the Trump ally said: "I don't know that we've ever had a witness come before our committee with less credibility than Michael Cohen."

Florida Republican lawmaker Matt Gaetz, a fierce defender of Mr Trump, has meanwhile deleted a tweet he posted threatening to reveal compromising information about Cohen's personal life.

Gaetz denied the tweet amounted to witness-tampering, contending it was "witness-testing".

Image source, Getty Images

"You've made it clear that you do not want the American people to hear what Mr Cohen has to say," says chairman Cummings to Republicans.

"But we're going to proceed.

"The American people can judge his credibility for themselves."

Republican Jim Jordan denied he was trying to stop Cohen's testimony, only that he wanted the committee's rules about submitting late evidence to be followed.