"They engage in violent protest tactics, which has included property damage and physical violence. They tend to be anti-capitalist and they are predominantly far-left and militant left, which includes anarchists, communists and socialists...
Scott Crow, described by CNN as 'a longtime Antifa organizer', argues that destroying property is not a form of violence. The groups have been associated with physical violence in public against police and against people whose political views its activists deem repugnant. Antifa activists used clubs and dyed liquids against the white supremacists in Charlottesville and caused property damage. In one incident, an apparent antifa supporter punched white supremacist Richard Spencer in the face as he was giving an impromptu street interview and on another occasion, in Berkeley, it was reported that some threw Molotov cocktails."
EDIT: And, if you're looking for individuals, these fine folks from the FBI most wanted domestic terrorists:
Is it really extremism to punch white supremacists/literal Nazi's though?
Click through this list:
https://en.wikipedia.org/wiki/List_of_designated_terrorist_o...
Plenty of leftist terror groups there. And most of them are probably on twitter, too. I have very specific interests on twitter, so I know PKK is there. And they advocate for violence.