Demonstrators flooded streets across the globe in public protests on Saturday, calling for action against gun violence. Hundreds of thousands of marchers turned out, in the most ambitious show of force yet from a student-driven movement that emerged after the recent massacre at a South Florida high school.At the main event in Washington, survivors of mass shootings, including the one in Florida, rallied a whooping crowd — “Welcome to the revolution,” said one of the student organizers — and spoke of communities that are disproportionately affected by gun violence. “It is normal to see flowers honoring the lives of black and brown youth that have lost their lives to a bullet,” Edna Chavez, 17, said of her South Los Angeles neighborhood.