US President Donald Trump signed an order Monday designating the left-wing Antifa movement as a domestic terrorist organization, the White House said, in a move sparked by the killing of right-wing ally Charlie Kirk.

Antifa is a shorthand term for "anti-fascist" used to describe diffuse far-left groups, and there have been questions since Trump first mooted the designation last week about how to define it.

Trump's order on Monday described Antifa as a "militarist, anarchist enterprise that explicitly calls for the overthrow of the United States Government" and was using "violence and terrorism" to suppress free speech.

"Because of the aforementioned pattern of political violence designed to suppress lawful political activity and obstruct the rule of law, I hereby designate Antifa as a 'domestic terrorist organization'," said the order.

But in an apparent nod to the questions about how to define Antifa, his order accused it of using "elaborate means and mechanisms to shield the identities of its operatives."

It used the same methods to hide its sources of funding, and recruit new members, the order said.

Trump's order also casts a net wide against the nebulous group. 

His order says US authorities can act against "any person claiming to act on behalf of Antifa, or for which Antifa or any person claiming to act on behalf of Antifa provided material support." 

Trump has repeatedly warned of a crackdown on left-wing groups since the assassination of activist Kirk, who was killed on September 10 at a Utah university campus, sparking right-wing rage.

US authorities have charged suspected shooter, 22-year-old Tyler Robinson, with murder. Robinson justified the attack by citing the "hatred" he accused Kirk of spreading, according to investigators. 

Rise in violence

But Trump has also threatened action against what he has called Antifa since his first term. 

He has blamed it for various wrongs from violence against police to being behind the US Capitol riot on January 6, 2021 that aimed to block Joe Biden's presidential election win.

Critics of the Republican president warn such a move could be used as a pretext to quash dissent and target political rivals.

While Kirk was a vocal conservative, the United States has seen violence targeting members of both political parties in recent years, amid a sharp rise in polarization and easy access to firearms.

Antifa -- whose name has roots in socialist groups in 1930s Germany that opposed Hitler -- has a track record of confronting right-wing groups and engaging in civil disobedience.

Antifa-aligned activists, often dressed entirely in black, protest against racism, far-right values and what they consider fascism, and say violent tactics are sometimes justified as self-defense. 

During Trump's first inauguration in January 2017 scores of black-clad, mask-wearing Antifa and other protestors smashed windows and burned a car in Washington.

Antifa was also involved in counter-protests to racist demonstrations in Charlottesville, Virginia later that year. 

by Danny KEMP