The areas of lower La Rambla and El Raval are to be enjoyed day and night but with "heads-up" caution literally. If Barcelona has a down-side where tourists need to be aware, these are it!
The beautiful Placa Reail, also with Gaudi's influence all over it, is probably the most risky place in the city...and more than just because of all the alternative types who hang out here. Drug dealers heavily frequent the area as do undercover officers...but don't take that as a green light for safety. People here survive by preying upon and taking advantage of others. My own NYC Spanish Harlem ghetto-based opinion of Placa Reail is that any tourist should not be deprived the opportunity to walk thru this courtyard simply to see and enjoy. But unless dining at one of the many restaurants or staying in a hotel, anyone that lingers here...day or night, is simply asking for trouble no two ways about it!
The Placa Reail crowd also heavily "works" La Rambla and more from just drug dealing. As written, Barcelonans flood the streets between 8-10p.m. Especially on La Rambla, the crowds are thick, the entertainers and over-all atmosphere are a good distraction for letting one's guard down and enjoying oneself...though it's highly risky. During my 10-day stay, I saw three seperate after-dark muggings/purse snatchings on La Rambla and in Placa Reail. (Almost puts NYC to shame!) I'm not going to lecture, but take heed and act accordingly. Pay attention, try to blend in, yada yada.
El Raval appears worse than it actually is. This large area is the community for new immigrants; mostly those from Morocco, other African countries while there's also a strong presence of people from the Dominican Republic. As you might expect, the area is more run down, less clean and apparent of a less primitive way of living compared to other parts of the city. The area is VERY interesting to see and take-in if your a multi-cultural freak like myself, but I recommend doing this in the day time. When the sun goes down, the area drastically changes as the dwellers simply take to the streets which might prove intimidating. A block south of Restaurante Pollo Rico, there were many prostitutes, street types and such that most wouldn't care to encounter, though I felt like it was another day back home!
The other "thieves" to be aware of along La Rambla are shopkeepers from India trying to hawk tourist junk for outrageous prices not including a more than doubled VAT tax. They're rude, aggressive - you know the rest. Once you've been in one shop, you've been in them all since they carried the same knock-off merchandise made at home in Asia. Foreign capitalism tainting authentic culture at it's best/worst.