The 6 January US Capitol attack by a mob of then-president Donald Trump's supporters trying to overturn his election loss featured harrowing violence resembling a medieval battlefield, a police officer hurt in the riots told a congressional panel.

At the Democratic-led House of Representatives investigatory committee's first hearing, the officer, Aquilino Gonell, described in prepared testimony being pummelled by rioters fired up by Mr Trump's false claims that the election was stolen from him through widespread voting fraud.

"What we were subjected to that day was like something from a medieval battlefield. We fought hand-to-hand and inch-by-inch to prevent an invasion of the Capitol by a violent mob intent on subverting our democratic process," added Officer Gonell, one of four police officers called to testify.

"The physical violence we experienced was horrific and devastating."

The nine-member panel was formed after Senate Republicans blocked the creation of an independent commission to investigate the attack. House Speaker Nancy Pelosi, a Democrat, named the committee's members. Its chairman is Democrat Bennie Thompson.

Officer Gonell and Harry Dunn, officers with the US Capitol police, and Michael Fanone and Daniel Hodges, officers with the District of Columbia police, were the panel's first witnesses.

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Officer Dunn, who is black, said in prepared testimony that rioters called him a racial slur while he was trying to defend the Capitol after he challenged their claims that no one had voted for Joe Biden by telling them that he himself was a Biden supporter.

Four people died on the day of the violence, including one rioter fatally shot by police and three others who died of natural causes. A Capitol police officer who had been attacked by protesters died the following day.

Two police officers who took part in the defence of the Capitol later took their own lives. More than a hundred police officers were injured.

Police were overwhelmed when hundreds of Trump supporters intent upon stopping Congress from formally certifying now-President Biden's 2020 election victory stormed the Capitol, smashing windows, fighting with officers and sending politicians and then-vice president Mike Pence scrambling for safety.

The riot followed Mr Trump's speech to supporters in which he repeated his false claims about voting fraud.

Ms Pelosi has called the attack "an attempt to overthrow the government."

Most House Republicans have fiercely opposed the creation of the committee, saying it is politically motivated by Democrats.

DC Metropolitan Police Department Officers Michael Fanone (L) and Daniel Hodges

Ms Pelosi last week rejected two of five Republicans chosen by House Minority Leader Kevin McCarthy for the panel amid concerns they would undermine the committee's integrity, leading Mr McCarthy to withdraw the three remaining Republicans names.

The committee will be include two Republicans - Liz Cheney and Adam Kinzinger - both of whom have denounced Mr Trump's false statements about the election and voted with the Democrats in January to impeach him.

Ms Cheney was stripped of her position in the House Republican leadership over her criticism of Mr Trump.

Mr McCarthy, a Trump ally, has derided Ms Cheney and Mr Kinzinger as "Pelosi Republicans."

"What we want to try to communicate during the hearing is what it was like to be on the front lines for these brave police officers, how vastly outnumbered they were, how well-militarised the members of the crowd were," House Intelligence Committee Chairman Adam Schiff, a panel member, told reporters yesterday.

Officer Fanone was pulled into the crowd of rioters, beaten, attacked with a Taser device and robbed of his badge, police radio and ammunition. As one rioter tried to pull his gun from its holster, Officer Fanone could hear him saying he planned to take it and kill him.

More than 535 people face criminal charges arising from the riot including four charged in the attack on Officer Fanone.