• Video report by ITV News Washington Correspondent Robert Moore

The historic impeachment trial against US president Donald Trump has begun with arguments already erupting over how to proceed as Democrats accused Republicans of trying to rig the process.

Mr Trump’s legacy - and the judgment of both parties in Congress - are on the line in this high stakes showdown in the Republican-controlled Senate - a process that could take weeks.

Even before proceedings got underway, sparks were flying with the Republicans keen to get the trial over within days, a move the Democrats branded a "national disgrace", claiming it was an attempt at a cover-up.

In the wake of protests, US Senate Majority leader Mitch McConnell abruptly changed his proposed rules, backing off the condensed two-day schedule to add a third for opening arguments.

Without comment, the Republican leader quietly submitted an amended proposal for the record, after meeting behind closed doors with senators as the trial opened.

He added the extra day and allowed House evidence to be included in the record.

  • Donald Trump's impeachment trial explained

Democrats had warned that the rules package from Mr Trump’s ally could force midnight sessions that would keep most Americans in the dark and create a sham proceeding.

Representative Adam Schiff said Senator McConnell's proposal to have a condensed, two-day calendar for each side to give opening statements was "not the process for a fair trial".

Republicans too had raised concerns about the timeline, among them Senator Susan Collins. Her spokesperson said the Maine senator saw the changes as a significant improvement.

Mr McConnell opened the chamber promising a “fair, even-handed” process — and warned that the Senate would stay in session until his proposed rules package was adopted.

“The president’s lawyers will finally receive a level playing field,” the Kentucky Republican said, contrasting it with the House impeachment inquiry.

The package diverges in some ways from the most recent impeachment trial, of Bill Clinton, by condensing opening arguments into two days for each side.

Mr Trump continues to dismiss the impeachment trial, telling reporters at the World Economic Forum Davos, Switzerland on Tuesday that it was a "hoax" and a "disgrace".

  • ITV News Washington Correspondent Robert Moore on the day's proceedings on Capitol Hill

It pushes votes to consider whether or not to hear from witnesses later in the process.

The Democrats has argued the prospect of middle-of-the-night proceedings, without allowing new witnesses or even the voluminous House records of the trial, will leave the public without crucial information about Mr Trump’s political pressure campaign on Ukraine and the White House’s obstruction of the House impeachment probe.

“The McConnell rules seem to be designed by President Trump for President Trump,” said the Senate Democratic leader, Chuck Schumer.

“This is a historic moment,” He added.

“The eyes of America are watching. Republican senators must rise to the occasion.”

Mr Trump continues to dismiss the impeachment trial, telling reporters at the World Economic Forum Davos, Switzerland on Tuesday that it was a "hoax" and a "disgrace".

House Democrats impeached the president last month on two charges: abuse of power by withholding US military aid to Ukraine as he pressed that country to investigate Democratic rival Joe Biden, and obstruction of Congress by refusing to comply with their investigation.

As the first day of the impeachment trial begins, Mr Trump touched down in Davos, Switzerland for his two-day visit to the World Economic Forum (WEF).

In the opening remarks at the summit, the president said that America’s economic turnaround has been “nothing short of spectacular”, as he sought to sell the United States to the global business community.

The Constitution gives the House the sole power to impeach a president and the Senate the final verdict by convening as the impeachment court for a trial.

After the four days of opening arguments — two days per side — senators will be allowed up to 16 hours for questions to the prosecution and defence, followed by four hours of debate. Only then will there be votes on calling other witnesses.

At the end of deliberations, the Senate would then vote on each impeachment article.

Mr McConnell had promised to set rules similar to the last trial, of President Bill Clinton in 1999, but his resolution diverged in key ways, which may leave some senators from both parties uneasy.

The Senate at sunrise. Credit: AP

Republican Senator Mitt Romney of Utah said in an email message to his constituents on Monday that the resolution put forth by Mr McConnell “overall, aligns closely with the rules package approved 100-0 during the Clinton trial”.

Mr Romney is among a small number of Republican senators who want to consider witness testimony and documents that weren’t part of the House impeachment investigation.

The Democrats’ Senate Minority Leader Chuck Schumer, seen here last week, has accused Republicans of trying to rush the impeachment proceedings. Credit: Julio Cortez/AP

In their filing on Monday, House prosecutors issued fresh demands for a fair trial in the Senate, which the Republicans control 53-47.

“President Trump asserts that his impeachment is a partisan ‘hoax.’ He is wrong,” the prosecutors wrote.

The White House document released on Monday says the two charges against the president don’t amount to impeachable offences. It asserts the impeachment inquiry, centered on Mr Trump’s request that Ukraine’s president open an investigation into Democratic rival Mr Biden, was never about finding the truth.

House Democrats in their initial court filing over the weekend called Mr Trump’s conduct the “worst nightmare” of the founding fathers who framed the Constitution.

But Mr Trump’s team contended Monday that even if Trump were to have abused his power in withholding the Ukraine military assistance, it would not be impeachable because it did not violate a specific criminal statute.