Security measures at the Venice Film Festival are tighter than ever, though some officials said the greatest threat this year may come from protesters rather than any kind of terrorist plot. Bolstered by greater numbers than in past years, security experts scan the beaches with metal detectors and examine screening locations for explosives before performances begin. A ring of steel surrounds the main venues, and metal detectors and strict bag checks upon entry are par for the course. Eric J. Lyman reports.