The violent anti-fascist protest movement known as "Antifa" is a loose collection of groups, networks and individuals who believe in active, aggressive opposition to far right movement, policies and individuals. Their ideology is rooted in the assumption that the Nazi party would never have been able to come to power in Germany if people had more aggressively fought them in the streets in the 1920s and 30s. Most Antifa come from the anarchist movement or from the far left, though since the 2016 presidential election, some people with more mainstream political backgrounds have also joined their ranks.

While most people who show up to counter or oppose conservative public events are peaceful demonstrators, when Antifa shows up, they can increase the chances that an event may turn violent. The current political climate increases the chances of violent confrontations at protests and rallies. Antifa have expanded their definition of fascist/fascism to include not just white supremacists and other extremists, but also many conservatives and supporters of President Trump. In Berkeley, for example, some Antifa members were captured on video harassing Trump supporters with no known extremist connections. Antifa have also falsely characterized some recent right wing rallies as "Nazi" events, even though they were not actually white supremacist in nature.

Violent Antifa tactics include throwing projectiles, the use of crowbars, homemade slingshots, metal chains, as well as water bottles and balloons filled with urine and feces. They have deployed noxious gases, pushed through police barricades, and attempted to exploit any perceived weakness in law enforcement presence.

Because there is no unifying body for Antifa, it is impossible to know how many "members" are currently active. Different localities have Antifa populations of different strengths, but Antifa members are also often  willing to travel hundreds of miles to events.