Washington DC - A federal appeals court ruled Friday that former president Donald Trump can be sued over the January 6, 2021, attack on the Capitol that saw his supporters attempt to thwart certification of his election loss to Joe Biden.

Trump could now face civil action over the violent clashes, which saw a mob overrun law enforcement at the nerve center of American democracy. More than 1,200 people have been arrested over the melee.

Two Capitol police officers, along with several Democratic lawmakers, sued Trump in 2021, alleging that he may have incited violence in his public comments to supporters before they descended on Capitol Hill.

Trump's legal team had argued that, as president, he had immunity for his actions, including comments telling his supporters to "fight like hell" as Congress prepared to certify his election defeat.

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"It is not that President Trump could not establish his entitlement to immunity... it is that he has not done so," said the ruling by a three-judge panel on the US Court of Appeals in Washington.

"When a first-term president opts to seek a second term, his campaign to win reelection is not an official presidential act," it said.

The ruling further explained the legal reasoning: "When a sitting president running for a second term... speaks at a campaign rally funded and organized by his reelection campaign committee, he is not carrying out the official duties of the presidency. He is acting as office-seeker, not office-holder."

Trump "recognized that he engaged in his campaign to win reelection — including his post-election efforts to alter the declared results in his favor — in his personal capacity as presidential candidate, not in his official capacity as sitting president," it said.

The Trump campaign did not immediately reply to a request for comment.