Washington (AFP) - The conspiracy that drove a mob to attack the US Capitol in January 2021 still poses a threat to American democracy, the head of the congressional committee tasked with investigating the deadly riot warned on Thursday evening at the panel's first public hearing.

In a live prime-time presentation, the committee offered the first conclusions from a year-long probe into the assault -- and outlined a deep-rooted and ongoing plot to undermine the US Constitution and overturn Donald Trump's election defeat.

The hearing served as an "opening statement" on the January 6 insurrection, laying out for the American public the causes of one of the darkest days in the history of US democracy.

The committee's Democratic chairman Bennie Thompson will said that his panel's work is about more than looking backwards, as US democracy "remains in danger."

"The conspiracy to thwart the will of the people is not over," warned Thompson.

"There are those in this country who thirst for power but have no love or respect for what makes America great: devotion to the Constitution, allegiance to the rule of law, our shared journey to build a more perfect Union."

Vice-chair Liz Cheney (R-WY) laid out in detail the coming weeks of hearings -- including Trump's "seven-part plan" to overturn the 2020 presidential election -- and offered a specific warning to the House Republicans who ousted her from leadership. "I say this to my Republican colleagues who are defending the indefensible. There will come a day when Donald Trump is gone, but your dishonor will remain,"

The panel began to demonstrate that the violence was part of a broader conspiracy by Trump and his inner circle to illegitimately cling to power, tearing up the Constitution and more than two centuries of peaceful transitions from one administration to the next.

"We will be revealing new details showing that the violence of January 6 was the result of a coordinated multi-step effort to overturn the results of the 2020 election and stop the transfer of power from Donald Trump to Joe Biden," a select committee aide said.

"And indeed that former president Donald Trump was at the center of that effort."

A slickly-produced two hours of television -- and five subsequent hearings over the coming weeks -- focused on Trump's role in the multi-pronged effort to return him to the Oval Office as an unelected president by disenfranchising millions of voters.

Trump has defiantly dismissed the probe as a baseless "witch hunt" -- but the public hearings were clearly on his mind Thursday as he launched into a largely false tirade on his social media platform, defending the insurrection as "the greatest movement in the history of our Country to Make America Great Again."

The case the committee began to make is that Trump laid the groundwork for the insurrection through months of lies about fraud in an election described by his own administration as the most secure ever.

His White House is accused of involvement in several potentially illegal schemes to aid the effort, including a plot to seize voting machines and another to appoint fake "alternative electors" from swing states who would ignore the will of their voters and hand victory to Trump.

'Chilling' Conspiracy

The committee is presented live testimony Thursday from Capitol Police officer Caroline Edwards and filmmaker Nick Quested, who interacted with members of the neofascist organization the Proud Boys on January 6 and in the days leading to the violence.

The hearing featured previously unseen video clips of the violence itself and excerpts from a trove of 1,000 interviews, including a "meaningful portion" of discussions with Trump's senior White House and campaign officials -- as well as members of his family. The committee played clips of Trump's daughter Ivanka and her husband Jared Kushner and former Trump aides who admitted that the president had been informed he lost the election.

Quested will testify Thursday about his experience shadowing members of the Proud Boys in the days leading up to January 6 and his interactions with them on the day itself.

The Emmy Award-winning director's evidence was crucial, said a committee aide, because he was on the scene during the first moments of violence against the Capitol Police and "all the chaos that ensued."

Court Of Public Opinion

Edwards, who was present at the breach of the first barricade, testified emotionally about sustaining head injuries in clashes with the far-right group, which saw its leader and four lieutenants charged on Monday with seditious conspiracy.

Outside the hearing, a number of Trump's most loyal counter-punchers are expected to circle the wagons on Capitol Hill, questioning any damning testimony and challenging the validity of the investigation.

"It is the most political and least legitimate committee in American history," the leader of the House Republican minority, Kevin McCarthy, told reporters at the Capitol.

In fact, Congress has wide-ranging oversight powers, and a Trump-appointed federal judge last month emphatically rejected Republicans' arguments that the committee is illegitimate and overtly partisan.

The committee has not confirmed its plans for after the initial slate of hearings, but at least one more presentation and a final report are expected in the fall.