Video report by ITV News Washington Correspondent Robert Moore
His comments came after the House Democrats read the formal charges from the well of the US Senate.
He faces two charges after the House voted to impeach him last month.
One, that he abused his presidential power by pressuring Ukraine to investigate Democratic rival Joe Biden, using military aid to the country as leverage.
He is also charged with obstructing Congress' ensuing probe.
The president insists he did nothing wrong, and he dismissed the trial anew on Thursday at the White House: "It's totally partisan. It’s a hoax."
The moment House prosecutors recited the charges against the US President
The US President told reporters everything was incorrect and that there was a "fake whistleblower".
He said: "So you have a perfect phone call, it was actually two phone calls.
"There were two phone calls, in fact probably the most nicest phone calls I've ever made to foreign leaders."
He added: "You had a fake whistleblower that wrote a report that bore no relationship to what was said. Everything was false."
He also tweeted on Thursday evening: "I just got impeached for making a perfect phone call."
Chief Justice John Roberts, who is to preside at the trial, administered the oath to more than 100 senators earlier, who will serve as jurors and swear to deliver "impartial justice".
House Speaker Nancy Pelosi said new evidence reinforces the need for senators to call witnesses.
She warned them not to become "all the president’s henchmen".
The trial, only the third such undertaking in American history, is unfolding at the start of the election year, a time of deep political division in the nation.
Four of the senators sitting in judgment on Mr Trump are running for the Democratic Party's nomination to challenge him in the fall.
Mr Trump's team expects acquittal with a Senate trial lasting no more than two weeks, according to senior administration officials.
That would be far shorter than the trial of President Bill Clinton, in 1999, or the first one, of President Andrew Johnson, in 1868 - both were acquitted.
The seven-member prosecution team is led by the chairmen of the House impeachment proceedings, Reps. Adam Schiff of the Intelligence Committee and Jerrold Nadler of the Judiciary Committee, two of Pelosi’s top lieutenants.