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Trump’s personal lawyer is a central figure in the Ukraine saga.

On the 25 July call, Trump asked Ukraine’s leader to co-ordinate with Giluiani on any inquiry into the Bidens, and Giuliani has already admitted to pushing Ukrainian officials to investigate unsubstantiated corruption allegations against Joe Biden.

He was to travel to Ukraine in May, but eventually decided against it. Earlier this year, two of his associates who helped connect Giuliani with Ukrainian officials were arrested on unrelated campaign finance charges.

Giuliani has also been accused of trying to discredit former Ukraine ambassador Marie Yovanovitch, who will testify publicly on Friday, while running a shadow foreign policy in Ukraine.

He has been subpoenaed for documents by impeachment investigators, but said he would not co-operate with Democrats.

George Kent, a state department official overseeing policy on Ukraine, is delivering his opening statement, describing an effort by Trump lawyer Rudy Giuliani to "smear" US officials in Ukraine.

"It was unexpected, and most unfortunate, to watch some Americans - including those who allied themselves with corrupt Ukrainians in pursuit of private agendas - launch attacks on dedicated public servants advancing US interests in Ukraine.

"In my opinion, those attacks undermined US and Ukrainian national interests and damaged our critical bilateral relationship."

"As a general principle, I do not believe the United States should ask other countries to engage in selective, politically associated investigations or prosecutions against opponents of those in power, because such selective actions undermine the rule of law regardless of the country," he added.

Image source, OLIVIER DOULIERY/AFP via Getty Images

Committee chairman Adam Schiff is calling on members of Congress and witnesses not to identify the anonymous whistleblower who first alerted Congress to the Trump-Zelensky call.

"We will not permit the outing of the whistleblower," Schiff warns Republicans in the committee after they asked for that person to be compelled to testify.

"I do not know the identity of the whistleblower and I am determined to make sure that identity is protected," he says.

Image source, Alex Wong/Getty Images

George Kent, one of two diplomats to testify today, oversees US policy on Ukraine. His formal title is Deputy Assistant Secretary in the US State Department's Bureau for European and Eurasian Affairs.

He was in regular communications with Bill Taylor, and heard about the 25 July call from Lt Col Alexander Vindman, who testified behind closed doors earlier.

He has also told lawmakers that he heard from officials in direct contact with Trump that the US president wanted Ukraine to publicly announce an investigation into the Bidens - which is key in tying all this back to Trump.

Republican Nunes winds down his opening statement by saying the whole hearing is a "televised theatrical performance staged by the Democrats".

"The main performance, the Russia hoax, has ended. And you have been cast in the low rent Ukrainian sequel," Nunes told the witnesses. "I'll conclude by noting the immense damage the politicised bureaucracy has done to Americans' faith in government."

"This spectacle is doing great damage to our country," he said.

"It's nothing more than an impeachment process in search of a crime."

As Republican Devin Nunes speaks, fellow party members respond with "Amen" and "hear hear".

Image source, Tom Williams/CQ-Roll Call, Inc via Getty Images

Republican Congressman Devin Nunes, the ranking chairman of the Intelligence Committee, denounced the Mueller investigation into alleged 2016 election meddling by Russia.

"After the spectacular implosion of their Russia hoax," he says, Democrats "turned on a dime and said the real malfeasance is Republicans dealing with Ukraine."

Schiff ends his statement by hearkening back to the nation's founders.

"Does the oath of office itself - requiring that our laws be faithfully executed, that our president defend a constitution that balances the powers of its branches, setting ambition against ambition so that we become no monarchy - still have meaning?"

"These are the questions we must ask and answer. Without rancour if we can, without delay regardless, and without party favor or prejudice if we are true to our responsibilities."

"Benjamin Franklin was asked what kind of a country America was to become, 'A Republic,' he answered, 'if you can keep it'.

"The fundamental issue raised by the impeachment inquiry into Donald J Trump is: Can we, keep it?"

Image source, Saul Loeb - Pool/Getty Images

In his opening statement, Schiff says that during the "infamous" phone call between Trump and Zelensky, "Trump complained that the US relationship with Ukraine had not been 'reciprocal'."

After Zelensky said his country was ready to purchase US missiles, Trump responded "I would like you to do us a favour, though".

Trump requested an investigation into the debunked "Crowdstrike" conspiracy theory - which claimed that Ukraine meddled in the 2016 election - and the Bidens.

"Neither of these investigations were in the US national interest," said Schiff.

"Both, however, were in Donald Trump’s personal interest, and in the interests of his 2020 re-election campaign."

BBC's Jon Sopel and Nick Bryant note the importance of the impeachment proceedings playing out today - a "once in a lifetime event"

Congressman Schiff's opening statement: Ukrainian president Zelensky "was a newcomer to politics" when he was first elected and sought to have a good relationship with the US.

Trump, he says, tried to "exploit that ally's vulnerability" for his own personal gain.

There's a lot going on in the impeachment inquiry. We’ve made it a bit easier to digest.

If you want a basic take on what's going on, this one's for you: A simple guide to the inquiry

Or, check out our 100, 300 and 800-word summary of the story if you want to go deeper

It's a political process to remove a president - watch our explainer here

What's the view from Trump country? Hear from residents of a town in West Virginia

How does a newly elected Democrat sell impeachment? Watch what voters think here

House Intelligence Committee Chairman Adam Schiff has just opened the first public stage of the investigation into Donald Trump.

Top diplomats William Taylor and George Kent have previously testified to lawmakers about alleged White House pressure on Ukraine to investigate ex-Vice-President Joe Biden - a political rival of Trump.

But this is the first stage of the impeachment process - which may remove Trump from office - to be televised live.

The Rupert Murdoch owned New York Post, which is famous for its over-the-top tabloid-style front pages has mocked today's impeachment hearing.

Intelligence Committee Chairman Adam Schiff is depicted as a circus ringleader firing a gag pistol and House Democratic Speaker of the House Nancy Pelosi is a clown wielding an over-sized gavel.

The president's eldest son, Donald Trump Jr, clearly approves since he retweeted the cover himself.

Republican Senator - and vocal Trump supporter - Lindsey Graham told Fox News that he won't be watching the hearings because they are "un-American".

"I think it is a threat to the presidency. I don’t want to legitimise it. It’s un-American. It denies the basics of due process."

On Wednesday, ahead of the hearings, Graham tweeted a number of questions about the whistleblower and Joe Biden's work in Ukraine.

"It is a huge day for President Trump," explains the BBC's Laura Trevelyan.

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Taylor, one of today's two witnesses, is the acting US ambassador to Ukraine.

He's delivered some of the most explosive testimony in this inquiry so far.

Taylor told a private last month that it was his "clear understanding" that the president had withheld military aid because he wanted Ukraine to investigate Joe Biden and his son on charges that have been widely discredited.

He also said that Trump's personal lawyer, Rudy Giuliani, was behind the drive to get Ukraine to announce an inquiry into the Bidens.

Now, Democrats are hoping Taylor can clearly explain it all to the public.

The BBC's Anthony Zurcher is at the Capitol as preparations for the hearing are under way.

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Intelligence Committee chairman Adam Schiff will begin the hearing at 10:00 local time (15:00 GMT).

He and the ranking Republican, Devin Nunes, will each deliver an opening statement before the witnesses are sworn in.

Bill Taylor, the top US diplomat in Ukraine, and George Kent, the deputy assistant secretary of state for European and Eurasian Affairs, will appear together.

From 10:30 local time, they will take questions from Schiff, Nunes and staff lawyers.

At noon, the rest of the committee will have their chance to question Taylor and Kent. Democratic and Republican legislators will take turns.

Marie Yovanovitch - the recalled US ambassador to Ukraine - will be next to testify publicly, appearing on Friday.

The BBC's Tara McKelvey is speaking to locals in Johnstown, Pennsylvania - a state that Trump won narrowly in 2016.