Unwilling to let the presumptive Republican nominee’s visit pass quietly, he created an event page on Facebook to organize a protest titled “Manda A Donald Trump A La Chingada” — Spanish for “Send Donald Trump to Hell.”

Two days later, Mr. Gonzales, and more than 100 other protesters who visited his page, arrived at the arena in San Jose where Mr. Trump would be speaking and jeered his supporters through a megaphone. The taunting at Thursday night’s rally turned violent, with some of Mr. Trump’s backers beaten and “Make America Great Again” hats set on fire.

“I saw he was coming and thought, ‘We got to go out and speak against him,’” said Mr. Gonzales, 31, who avoided the violence that ensued. “He’s obviously not representing what we’re all about here in San Jose. San Jose is very based on immigrants and stuff like that.”…

Joshua Tucker, a New York University political science professor who studies social media and political participation, said protest movements that develop on their own, rather than ones organized by a single leader, can be more chaotic because people are reacting to what happens on the street and are given little guidance on decorum.