Democrats have slammed Donald Trump's impeachment trial as unfair and rigged, with the potential for proceedings to acquit the President without actually proving his innocence.

Rules governing the process make it “impossible to have a fair trial” and merely aid Mr Trump's cover-up of the truth, warned Adam Schiff, lead manager of the Senate impeachment trial.

It fails to guarantee witnesses or that evidence gathered by investigators would be allowed into the record, he said.

"It will not prove the president innocent – it will merely prove the Senate guilty of working with the president to obstruct the truth from coming out."

Read: Donald Trump’s impeachment trial: How will it work?

Just hours before the trial was due to begin on Tuesday, he along with several other House impeachment managers released a statement denouncing rules proposed by Senate majority leader Mitch McConnell, which set up a swift impeachment trial for Mr Trump.

The trial guidelines unveiled on Monday would give House Democratic prosecutors and Mr Trump's lawyers 48 hours, evenly split, to present their arguments over a maximum of four days.

“A White House-driven and rigged process, with a truncated schedule designed to go late into the night and further conceal the President’s misconduct, is not what the American people expect or deserve,” the House impeachment managers said.

Meanwhile, Mr Trump was in Switzerland to address the annual World Economic Forum. He used his speech to hit out at environmental "prophets of doom".

Democrats blast Donald Trump impeachment trial rules_1
Inset: President Donald Trump at the World Economic Forum. Photo: AAP

Mr Trump announced the US would join an existing initiative to plant one trillion trees, but also spoke at length about the economic importance of oil and gas and called climate change activists "pessimistic" and the "heirs of yesterday's foolish fortune tellers".

Back home, House managers responded to fiery accusations by Mr Trump's legal team earlier this week that Democrats were "focus-group testing various charges for weeks" and that "all that House Democrats have succeeded in proving is that the President did absolutely nothing wrong".

Minutes away from debating amendments to the trial's rules, House managers released a statement saying the brief filed by Mr Trump's lawyers "confirms that his misconduct is indefensible".

“President Trump’s lengthy brief to the Senate is heavy on rhetoric and procedural grievances, but entirely lacks a legitimate defense of his misconduct.

“It is clear from his response that President Trump would rather discuss anything other than what he actually did.”

Under the current resolution, Mr Trump's lawyers could move early in the proceedings to ask senators to dismiss all charges, a senior Republican leadership aide said, a motion that would likely fall short of the support needed to succeed.

Democrats blast Donald Trump impeachment trial rules_2
Inset: Senate Majority Leader Mitch McConnell arrives at the Senate for the start of the impeachment trial. Photo: AAP

Senate minority leader Chuck Schumer said the trial would “result in a rushed trial with little evidence, in the dark of night".

House Speaker Nancy Pelosi, in a statement, said the Republicans' plan would force the trial into the "dark of night" and accused McConnell of choosing "a cover-up for the president, rather than honour his oath to the Constitution."

The trial is expected to continue six days a week, Monday through Saturday, until at least the end of January.

In December, the US Democrats charged the president with abuse of power and obstruction of congress for trying to pressure Ukrainian President Volodymyr Zelensky to investigate his political rival, Joe Biden and his son Hunter.

If Mr Trump is convicted, Vice President Mike Pence would become president for the remainder of Mr Trump’s term, which ends on January 20, 2021.

The Senate has the power to remove Mr Trump from office if more than two-thirds of them agree to do so.

But there’s a catch – given the Senate is dominated by Republicans, it is very unlikely they will convict him.

-with AAP