Late Saturday morning, a peaceful demonstration grew around the fenced-off Alamo Square Park under a bright sun and a cloudless sky. Families toting young children and bandanna-masked anarchists marched next to each other, chanting and hoisting signs such as “If you’re going to San Francisco, DON’T BE A NAZI!” “Fight White Supremacy!” and “More Sex Less Hate.” A phalanx of police officers looked on impassively.