President Donald Trump on Monday pardoned nearly everyone criminally charged with participating in the January 6, 2021, attack on the US Capitol, in a show of solidarity with supporters who stormed the seat of American power in his name.

The far-reaching action also cuts short the sentences of 14 members of the Proud Boys and Oath Keepers organisations, including some who were convicted of seditious conspiracy.

The document also directs the US attorney general to drop all pending cases related to the riot.

"What they've done to these people is outrageous," Trump, a Republican, told reporters in the Oval Office.

"These are the hostages - approximately 1,500 people - for a pardon, full pardon," he said at a signing ceremony shortly after arriving at the White House.

"We hope they come out tonight frankly," he said. "They're expecting it."

Trump, whose first term as president ended under the cloud of the Capitol assault, has repeatedly played down the unprecedented violence of January 6, even going so far as to describe it as a "day of love."

More than 140 police officers were injured in hours of clashes with rioters wielding flagpoles, baseball bats, hockey sticks and other makeshift weapons along with Tasers and canisters of bear spray.

The assault on the Capitol followed a fiery speech by then-president Trump to tens of thousands of his supporters near the White House in which he repeated his false claims that he won the 2020 race. He then encouraged the crowd to march on Congress.

Trump was charged by special counsel Jack Smith with conspiring to overturn the results of the 2020 election.

But the case never made it to trial, and was dropped under the Justice Department's policy of not prosecuting a sitting president.

According to the latest figures from the US Attorney's Office for the District of Columbia, 1,583 people have been charged in connection with the Capitol siege, including 608 accused of assaulting, resisting or impeding law enforcement officers.

A lawyer for the former leader of the Proud Boys group, Enrique Tarrio, said he expected his client to be released from federal prison.

Tarrio is serving a 22-year sentence, the longest of anyone criminally charged in the riot, for seditious conspiracy. He was found guilty of plotting to violently oppose the transfer of power after the 2020 election.

A lawyer for Tarrio's co-defendant, Joe Biggs, said he was also told by an intermediary that Biggs is being processed for release. Biggs, who held a senior post in the Proud Boys, was also convicted of seditious conspiracy and sentenced to 17 years in prison.

The US Constitution gives presidents broad pardon power and there is no legal mechanism for challenging a presidential pardon.

Biden, before leaving office on Monday, issued pre-emptive pardons to former Covid pandemic advisor Anthony Fauci, retired general Mark Milley and close family members to shield them from "politically-motivated prosecutions" by the Trump administration.

Biden gave similar pardons to former Republican lawmaker Liz Cheney and other members of the congressional committee that investigated the January 6 attack on the Capitol.

Just minutes before Trump was sworn in, Biden announced he was issuing pardons to his brother James Biden, James's wife Sara Jones Biden, his sister Valerie Biden Owens, Valerie's husband John Owens, and his brother Francis Biden.

"My family has been subjected to unrelenting attacks and threats, motivated solely by a desire to hurt me -- the worst kind of partisan politics," Biden said.

"Unfortunately, I have no reason to believe these attacks will end." (Agencies)