House Speaker Nancy Pelosi told reporters on Wednesday that she was "very prayerful, thoughtful and actually saddened today".

The top Democrat said Trump does not understand that "he is not above the law and that he will be held accountable".

"We have a responsibility to keep as custodians of the Constitution," she added. "We are defenders of our democracy."

Republicans are also permitted to request witnesses, but they must be approved by Chairman Adam Schiff - a Democrat.

Republicans wanted to hear from Hunter Biden, the son of former vice-president Joes Biden, and the anonymous whistleblower who alerted Congress to the Trump-Zelensky call where a "favour" was discussed.

Schiff rejected those requests saying the committee would not serve "as a vehicle to undertake the same sham investigations into the Bidens or 2016 that the president pressed Ukraine to conduct for his personal political benefit".

One of two witnesses appearing today, Acting Ambassador to Ukraine Bill Taylor, has arrived on Capitol Hill. The hearing is due to begin in less than an hour.

Image source, Getty Images

There are at least two.

Whistleblower #1 wrote to senators in August expressing concern over Trump's July phone call with the Ukrainian president.

The individual also alleged that the White House had acted to "lock down" all details of the phone call between Trump and Zelensky.

All we know about #1 is that he or she is a US intelligence officer - or at least was when the letter was written - but does not work in the White House.

US media have reported that the person is a CIA officer.

Lawyers for the whistleblower then said a second person from the intelligence community had come forward (reportedly with direct knowledge of the allegations related to the Trump-Zelensky call).

Meanwhile, Trump has called whistleblower #1 a “disgrace to our country” and said they should be named - despite concerns for the person’s safety.

Both Republicans and Democrats have used inflammatory language throughout this process.

Republicans accuse their counterparts of trying to stage a "coup" against Trump since the day he took office.

House Intelligence Committee Chairman Adam Schiff has previously described the Trump-Zelensky call as "a classic Mafia-like shakedown".

Democrat Eric Swalwell, a member of the intelligence committee which is holding the hearings, used similar language on Tuesday saying: "It’s time for these witnesses to go before the American people and lay out what they saw in this extortion scheme."

Impeachment is the first part - the charges - of a two-stage political process by which Congress can remove a president from office.

If the House of Representatives votes to pass articles of impeachment, the Senate is forced to hold a trial.

A Senate vote requires a two-thirds majority to convict - unlikely in this case, given that Mr Trump's party controls the chamber.

Only two US presidents in history - Bill Clinton and Andrew Johnson - have been impeached but neither was convicted and removed.

President Nixon resigned before he could have been impeached.

This morning, the president criticised his opponents by quoting his favourite news programme, Fox and Friends.

"The Democrats have stacked the deck against President Trump and the Republicans. They have leaked out everything," Trump tweeted, citing the morning TV show.

Since Tuesday evening he has been sharing quotes from his supporters speaking on the Fox network.

His latest tweet reiterates: "READ THE TRANSCRIPT!"

Image source, Getty Images

Ukrainian President Volodymyr Zelensky told reporters in October there had been “no blackmail” in the call with Donald Trump that led to this impeachment probe.

He said the purpose of the conversation was to arrange a meeting with Trump, and there had been no "conditions" from the American side.

Zelensky also said he did not believe US-Ukraine relations would be affected by the impeachment inquiry.

Read more here.

Welch, West Virginia, is in McDowell County, where US President Donald Trump received more than 70% of the vote in the 2016 election. The BBC spoke to locals about the ongoing impeachment inquiry.

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The Democratic Chairman of the House Intelligence Committee, Adam Schiff, has set the rules for today's hearing.

Schiff has promised a hearing that is "fair to the president". To keep the panel from turning into a free-for-all, he has given himself and his Republican counterpart Devin Nunes 45 minutes each to ask questions.

During this time, they may allow their staff lawyers to take over questioning.

After that, the other 12 Democrats and eight Republicans on the committee will be given five minutes each for questions.

This is the first time the American public will hear testimony from the US officials at the heart of inquiry into whether Trump abused his presidential powers.

A mystery whistleblower, a comedian turned head of state, and the president of the United States.

These are some of the main players in a story that is becoming ever more complex - and could see Donald Trump removed from office.

Catch up on the cast of characters here.

We're in the midst of an impeachment inquiry. But how does this process actually work?

Image source, Getty Images

You’ve got a couple of hours to catch up with all things Trump impeachment - and we’ve made it easy for you.

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

Image source, Drew Angerer/Getty Images

Ahead of the hearing, Democrats and Republicans prepped behind closed doors on Tuesday night.

Democrats are expected to focus on the “simple story” of a president who abused his powers.

Republicans meanwhile continue to defend Trump and have accused Democrats of obscuring the facts.

The BBC's Anthony Zurcher explains that the president's party will say Trump did not pressure Ukraine for personal gain, but because he had legitimate concerns about corruption - and aid was eventually released. They may dismiss the witnesses as unelected bureaucrats and undermine the credibility of the first anonymous whistleblower.

Read more of Anthony's analysis here.

Image source, Photo by Chip Somodevilla/Getty Images

Trump is accused of breaking the law by pressuring Ukraine's leader to dig up damaging information on a political rival.

In July, he urged his Ukrainian counterpart to investigate one of the frontrunners to take him on in next year's presidential election. This matters because it is illegal to ask foreign entities for help in winning a US election.

The impeachment inquiry could see the president eventually removed from office.

But there is a fierce debate about whether Mr Trump broke the law or committed an impeachable offence - he himself says he has done nothing wrong.

Want more detail? Read the full story here.

Welcome to our live coverage of the first public hearings in the impeachment inquiry into President Donald Trump.

We’ll get to see House of Representatives lawmakers question two top diplomats - Bill Taylor and George Kent - later this morning.

At the heart of it all: an investigation into whether the president abused power by pressuring Ukraine to investigate a political rival.

We’ll keep you updated on the latest happenings on Capitol Hill and what it all means here.