From Stratfor:

Anarchists, along with Marxists, Maoists and anarcho-syndicalists, are usually among the most visible, vocal and violent elements that participate in antifa protests. Their respective black, red, and red-and-black flags are a ubiquitous sight at antifa gatherings, and it is no coincidence that many participants wear clothing with the same colors. The anarchist Black Bloc contingent, for example, is usually conspicuous at protests due to its dress, flags and the violence it leaves in its wake.