The Donald Trump impeachment trial has begun in the US Senate.
It comes after the House of Representatives charged Mr Trump with abuse of power and obstruction of Congress
Mr Trump has become the third US president to face a Senate trial, following on from Andrew Johnson and Bill Clinton.
The current president is not expected to be removed from office as the Senate is currently controlled by Republicans.
The lead-up to the trial has been dominated by a row over whether Democrats would be allowed call new witnesses and use new evidence to support their accusation that Mr Trump attempted to pressure Ukraine into investigating a political rival - Democrat Joe Biden - by withholding military aid.
Proceedings formally got underway today with a debate over the rules of the trial.
In an opening address to senators, Democrat Adam Schiff - one of the House's 'impeachment managers' - claimed the misconduct set out in the articles of impeachment is "the most serious ever charged against a president".
“The misconduct set out in those articles is the most serious ever charged against a president.” - @RepAdamSchiff begins the Senate trial in the impeachment of Donald J. Trump pic.twitter.com/j53Kchw6ib
— House Intelligence Committee (@HouseIntel) January 21, 2020
He argued: "As a matter of history and precedent, it would be wrong to assert that the Senate is unable to obtain and review new evidence during a Senate trial - regardless of why the evidence was not produced in the House.
"You can and should insist on receiving all the evidence so you can render impartial justice and can earn the confidence of the public in the Senate's willingness to hold a fair trial".
Pat Cipollone, White House counsel, said his team was proud to be representing the US president.
He argued that the proposed rules for the trial were modelled on the rules set out for the Clinton impeachment trial.
He added: "We believe that once you hear those initial presentations, the only conclusion will be that the President has done absolutely nothing wrong and that these articles of impeachment do not begin to approach the standard required by the Constitution."
Procedure
A number of 'impeachment managers' will take part in the trial for the Democrats, while Mr Trump has his own legal team in place.
The chief justice of the US Supreme Court - John Roberts - is presiding over the process.
After four days of opening arguments, senators will be allowed up to 16 hours for questions to the prosecution and defence - followed by four hours of debate.
A vote on whether additional witnesses will be called will then take place.
Senators will sit for six days a week - taking only Sundays off - until the trial ends.
Mr Trump is not in Washington for the beginning of proceedings, as he's attending the World Economic Forum in Davos.
