For the last 2 years the major danger in Barcelona has been "Car attacks". Tourists are warned on some embassys' websites but not all. On average, the attack happens every day, sometimes several times a day. It occurs on the large streets leading outside Barcelona (eg Avenida Meridiana) but is also occurring now on more central boulevards like the Avenida Diagonal. The procedure is simple and very efficient. It targets tourists driving a foreign car and leaving Barcelona (their car may look more heavily loaded than Barcelona residents'). At some moment, these tourists are warned that a rear tyre of their car is getting flat. The warners are several "friendly" men in a car and another man on a motorbike. The tyre was previously punctured, when the tourist car was stopped at a traffic light. While the tourist stops and pulls all the luggages fom the trunk, all the valuables left at the front of the car are stolen. It is not limited to small bags, nor avoided by letting the doors shut (if unlocked). To help the thieves take the heavier stolen things away, the motorbike man comes back and draws the tourist's attention by gesticulating some information about a nearby workshop that could help. Stopping in an open space like a public place with many ordinary citizens walking and being able to see what is happening is absolutely useless, for the robbers are several, have a brilliant logistics and are so expert that they can remain unnoticed by someone still in contact with the car. Another reason why the locals will not react is that the situation is truly dangerous. A spanish friend had the tyres of her car punctured after she had advised Swiss tourists, whose tyre was getting flat, to drive without stopping until the next petrol station. More regrettable is the lack of warning from the state of Catalunya and the Mayor of Barcelona. There should be a warning in all the hotels and car parks where the tourist drivers go ! Finally, there is little hope that the stolen objects with no commercial value will be brought back to Barcelona's "found objects" service.

Be alert, as in any large city. The central part of Barcelona, especially always crowded La Rambla, is renowned for petty crime, such as pickpocketing and bag snatching, so be watchful when sightseeing, taking pictures or watching street artists. Pickpockets are known to be crafty when devising ruses to dupe tourists out of their money.

The safest thing to do is to not respond to anybody unknown to you who comes up to you on the street, no matter how safe they appear to be. Seemingly benign people can turn dangerous in a heartbeat. Criminals are known to pretend to be hurt, and even dress up in women's clothing, asking for assistance in the street. When the unwary tourist approaches them, however, they will rob him and run off.

Don’t take any valuables with you, keep your bags closed and don’t carry money in your pockets. Also be cautious when using public transportation. In restaurants always keep your bag or backpack beside you or between your legs on the floor so you can constantly see it. Overall, Barcelona is a safe city where violent crimes are rare. In restaurants, do not hang your coat over the back of a chair if it contains a wallet.

 One scam to be wary of, is when on the street you may be told or find that a bird has pooped on you - beware the good samaritan. A number of cases have been reported on the web of people being sprayed with bird muck like substance. The good samaritan who comes to your rescue with an offer of water, not only helps clean your jacket etc but he also helps himself to your belongings. 

 Be careful of people with a clipboard asking you to sign a petition. They work in pairs so as you are reading the petition and talking to them their partner comes into try and pickpocket you.

More recently crooks pretending to be undercover policemen have asked tourists for identification, or have accused them of collaborating with known criminals and have threatened them with arrest.  The 'police' ask for identification, particularly passports, and credit cards or wallets which they then disappear with.  As of April 2008 there are plenty of police patrolling tourist areas so tourists should be familiar with their uniforms.  Unless you are drunk, disorderly or involved in some sort of crime it's very unlikely a policeman will ask for your ID.