The first live, televised hearing has begun in the impeachment inquiry into US President Donald Trump.

A series of government officials are taking to the stand in the public hearings as Trump faces the most perilous challenge of his three-year presidency.

Watch the proceedings live:

Adam Schiff, the Californian congressman overseeing the historic inquiry said there were "few actions as consequential as the impeachment of a president." 

"The questions presented by this impeachment inquiry are whether President Trump sought to exploit (Ukraine's) vulnerability and invite Ukraine's interference in our elections," Schiff said.

"If this is not impeachable conduct, what is?"

Trump denies trying to pressure Ukraine into investigating former vice-president Joe Biden, labelling the inquiry "corrupt", "illegal,"  and a "phony showtrial".

After opening remarks by the Chairman of the House of Representatives Intelligence Committee, diplomats Bill Taylor and George Kent will be the first to testify.

Speaking minutes before the start of the hearings, Nancy Pelosi, the Democratic speaker of the House, said the probe was necessary to show Trump he can't do "whatever he wants."

"That he is not above the law," Pelosi said. "And that he will be held accountable." 

Democrats who control the House plan to prove over several weeks of hearings that Trump abused his office by asking Ukraine to conduct a politically motivated investigation into his potential 2020 Democratic president rival Joe Biden.

The investigation threatens to make Trump only the third US president to be impeached, after Andrew Johnson in 1868 and Bill Clinton in 1998, although to be removed from office he would need to be convicted by the Republican-led Senate.

Neither Johnson or Clinton was convicted and removed. But in 1974 Richard Nixon resigned in the face of certain impeachment and removal from office for the Watergate scandal.