While Mr. Trump has blamed paid “disrupters” and operatives from Hillary Clinton’s and Senator Bernie Sanders’s campaigns for such protests in the past, the violence at this rally and the protest that inspired it appeared spontaneous and not directed by Democrats or other groups. Instead, it was more the product of independent local efforts that caught fire through social media.

Such episodes, like a protest in April in Burlingame near San Francisco and at an abruptly canceled March rally in Chicago, represent some of the most significant unrest during a presidential contest since 1968, when violence erupted in the streets outside the Democratic National Convention in Chicago at the height of the Vietnam War.

Mr. Gonzales’s group was one of many that arrived on Thursday to express disapproval of Mr. Trump, who has angered many critics with proposals they consider to be anti-immigrant. While protest organizers and attendees said that violence was not their intention, clashes with Mr. Trump’s supporters have become increasingly common.

The disturbances are the result of a combustible mix: the passion of the anti-Trump movement and, on the other side, the often fiery oratory of Mr. Trump himself, who at a rally this year in Cedar Rapids, Iowa, went so far as to urge his supporters to “knock the crap” out of protesters at the event.

On Friday, Mr. Trump, who has taken some measures to quell violence inside his events, lashed out at those who created the chaos at Thursday’s event. At a rally in Redding, Calif., Mr. Trump said his supporters “got accosted by a bunch of thugs burning the American flag.”