Barcelona has a significant pickpocketing problem — worse than most European capitals. Las Ramblas is notorious: walking the famous boulevard with a visible phone or open bag invites theft. The Metro (especially line 3), Barceloneta beach, and the Gothic Quarter are other hotspots. Use anti-theft backpacks, keep phones in pockets, and don't put bags on table backs in restaurants.
Beyond pickpocketing, Barcelona is safe. The 'mustard scam' (someone squirts mustard on you, offers to help clean, and an accomplice takes your bag) operates near tourist sites. Violent crime against tourists is rare. The emergency number is 112; tourist police (Mossos d'Esquadra) are visible at major sites.