Working with tourists all around the world you could almost make a slogan: “Everyone has lost something in Barcelona.” It might not be completely true, but many people have experience to lose their wallet or other valuables while in Barcelona. Therefore we recommend all visitors to take good care of their valuables, especially as they walk along La Rambla and its neighborhood, and while using public transportation.
After sunset you should make sure not to walk alone, and it could be a smart thing to pay extra attention to your values at this time of the day as well.
Some useful telephone numbers:
Police: 092
Emergencies: 061