In a memo to law enforcement and prosecutors released by the Department of Justice, Barr said alleged extremists had "engaged in indefensible acts of violence designed to undermine public order," including attacking police officers, damaging property and threatening innocent people.
Protests have spread nationwide over George Floyd’s death in police custody last month and the deaths of other African Americans at the hands of police.
Although largely peaceful, some demonstrators have turned violent, which President Donald Trump and his allies have blamed on left-wing extremists among the protesters.
Barr said the extremists "profess a variety of ideologies."
"Some pretend to profess a message of freedom and progress, but they are in fact forces of anarchy, destruction, and coercion," Barr said.