Updated

Attorney General William Barr said Sunday that the Department of Justice will treat violence by individuals associated with Antifa as domestic terrorism in a statement that condemned the far-left group and asserted that protests against police brutality and racial inequality following George Floyd's death have "been hijacked."

Such demonstrations in cities nationwide have turned into violent riots complete with looting, attacks against police and arson.

Barr's statement came after President Trump earlier Sunday said he would designate Antifa as a terrorist organization. Antifa -- short for "anti-fascist" -- has very little central organization but violent left-wing protesters, particularly in places like Portland, Ore., often act under the Antifa banner.

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"With the rioting that is occurring in many of our cities around the country, the voices of peaceful and legitimate protests have been hijacked by violent radical elements. Groups of outside radicals and agitators are exploiting the situation to pursue their own separate, violent, and extremist agenda," Barr said.

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He continued: "It is time to stop watching the violence and to confront and stop it. The continued violence and destruction of property endangers the lives and livelihoods of others, and interferes with the rights of peaceful protestors, as well as all other citizens... The violence instigated and carried out by Antifa and other similar groups in connection with the rioting is domestic terrorism and will be treated accordingly."

Kupec also tweeted: "As a reminder, it is a federal crime to cross state lines or to use interstate facilities to incite or participate in violent rioting. We WILL be enforcing these laws."

The American Civil Liberties Union (ACLU) condemned the move by the Trump administration to designate Antifa as a terrorist group, saying the decision showed "terrorism is an inherently political label, easily abused and misused."

"There is no legal authority for designating a domestic group," the ACLU added. "Any such designation would raise significant due process and First Amendment concerns."

Antifa has been blamed for causing at least some of the violence that has happened in the wake of the death of George Floyd, the unarmed black man who died May 25 while in police custody in Minneapolis, Minn., after an officer kneeled on his neck for more than 8 minutes in an incident caught on video.

It's not clear that Antifa is behind all or even most of the violence in the widespread protests. But, the left-wing group known for bringing radicals across state lines to instigate unrest has been a target of Republican politicians who have made clear they stand with peaceful protesters they say are justifiably angry about Floyd's death and other racially charged interactions between African-Americans and police in recent months.

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"It’s ANTIFA and the Radical Left. Don’t lay the blame on others!" Trump tweeted on Saturday.

Democrats, on the other hand, have blamed white supremacists for allegedly infiltrating the protests and instigating violence.

Minneapolis Mayor Jacob Frey said Saturday that officials think "white supremacists" and "out-of-state instigators" could be behind the protests, while Minnesota Gov. Tim Walz said the majority of the protestors arrested were from outside Minneapolis and sought to take advantage of the chaos.

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However, a report by KARE 11 showed "about 86 percent" of the recent 36 arrests by Minneapolis-based police agencies listed their address in Minnesota, and that they live in Minneapolis or the metro area, according to data the outlet analyzed from the Hennepin County Jail's roster.

Barr continued in his Sunday statement to say that the FBI Joint Terrorism Task Force (JTTF) would be involved in identifying "criminal organizers and instigators" and that federal agencies will work toward "apprehending and charging the violent radical agitators who have hijacked peaceful protest and are engaged in violations of federal law."

In reaction to the violence, a number of U.S. cities have instituted curfews for Sunday night. These include Chicago, Ill.; Cincinnati, Ohio; Cleveland, Ohio; Columbia, S.C.; Columbus, Ohio.; Dallas, Texas; Denver, Colo.; Louisville, Ky.; Los Angeles, Calif.; Miami, Fla.; Myrtle Beach, S.C.; Philadelphia, Pa.; Richmond, Va.; Rochester, N.Y.; Salem, Ore.; Salt Lake City, Utah.; San Antonio, Texas; San Francisco, Calif.; Seattle, Wash.; and Syracuse, N.Y.

Fox News' Adam Shaw and Ronn Blitzer contributed to this report.