Former US president Donald Trump can be sued by police and others injured in the January 6, 2021 attack by his supporters on the US Capitol, the Justice Department said in a court filing Thursday.

In an official legal opinion submitted to federal court in Washington, the department said although a president has absolute immunity for his official acts in office, he can be sued for acts judged clearly outside of his official duties.

Trump, who was sued alongside other key actors accused of inciting the attack -- which took place two weeks before he was to leave the White House -- claimed protection from being sued by his absolute immunity.

Trump, the Justice Department said, claimed immunity because his pronouncements up to and during the attack amounted to "speech on matters of public concern," which is within his official duties, even if that speech can be seen as inciting violence.

The Justice Department said the court should reject Trump's categorical argument.

"As the nation's leader and head of state, the president has 'an extraordinary power to speak to his fellow citizens and on their behalf,'" the department said.

"But that traditional function is one of public communication and persuasion, not incitement of imminent private violence," it said.

The department said it was not commenting on the merits of the case -- whether Trump did or did not incite violence -- but only indicated that doing so was not covered by his presidential immunity.