The announcement followed heightened political tensions in the U.S. after the fatal shooting of conservative activist Charlie Kirk.

The president posted the Truth Social message in the early hours of Thursday UK time while he and First Lady Melania Trump were scheduled to meet King Charles III and Queen Camilla.

The White House confirmed to CNN that the move is one of several actions the administration plans to take to address what it describes as left-wing organizations that fuel political violence.

A White House official told the network the designation is part of a broader effort by the administration to confront groups it views as responsible for violent unrest.

Legal and practical questions remain about the mechanics of the designation.

Antifa is a loosely organized movement without a centralized leadership structure or a formal membership roster, and legal experts have previously noted that U.S. terrorism-designation authorities are typically used to label foreign organizations.

Those limits have prompted debate among lawmakers and legal analysts about how a domestic or decentralized movement might be targeted under existing statutes.

The designation announcement drew immediate attention from commentators and independent reporters who have tracked Antifa’s activities.

Independent journalist Andy Ngo, who has reported extensively on the movement, has documented posts and messaging on social media platforms he says demonstrate calls for violence.

In a post highlighted by his followers, Ngo captured a comment he attributed to Antifa supporters: “”You’ve been training your whole life for this, guys.””

Ngo and other conservative commentators have cited social-media posts they say encouraged violence against specific conservative figures, including Charlie Kirk.

Those claims have been a focus of conservative outlets and activists who argue the movement’s rhetoric and online organizing contributed to an atmosphere of political violence leading up to the Kirk killing.

Antifa’s critics point to episodes of violent protest and clashes with law enforcement in several U.S. cities over the past decade.

Organizers and scholars who study protests note Antifa is a label applied to a range of far-left and anti-fascist activists, not a single hierarchical organization.

That diffuse structure has complicated efforts to pursue legal remedies against the movement under statutes designed for formal terrorist organizations.

The president’s announcement prompted immediate responses from lawmakers and commentators.

Several House Republicans expressed support for Trump’s move and called for federal investigations into groups and funding sources the administration alleges are linked to violent activity.

Other legal experts and civil liberties advocates warned that designating a domestic movement could raise First Amendment and due process concerns and may face legal challenges if the administration attempts to impose penalties reserved for formal terrorist organizations.

The decision places federal authorities and legal counsel in the position of defining what constitutes Antifa for enforcement purposes, if and when the administration moves beyond the public declaration.

It also raises questions about which federal tools—criminal prosecutions, sanctions, civil actions, or regulatory measures—would be used to pursue individuals or organizations the administration identifies as supporters or enablers.

The announcement comes amid a broader conservative push to hold left-wing activist groups and philanthropic funding to greater scrutiny.

The president’s post said he would “strongly recommend” investigations into entities that fund Antifa activities and urged adherence to “the highest legal standards and practices.”

Officials have not yet released a formal legal framework or list of organizations and individuals that would be covered under the announcement.

Federal agencies typically require interagency review and legal vetting before implementing designations with potential criminal or civil consequences.

The administration’s next steps, including any formal orders, regulations or referrals to the Department of Justice, have not been publicly detailed as of Thursday.