Here's a word of advice if you are advised to take your passport to register with the "police" (I'm not sure where I went either). Anyway, you'll be sent to an office where they handle visas, so you can extend for, let's say, the entire year (the initial visa you get will only be valid for a short time). It has been several (wow, many) years since I did this but here's my advice if it's still the same...DON'T STAND IN THE LONG LINE! It's very possible that things have changed but when I went, there was a line of approximately 200 people waiting to "apply, process", something having to do with immigrant/work visas. This line is not for you, don't bother with it. I gave the same advice to a friend and, low and behold, she stood in the line for hours before she asked someone if she needed to stand in it. You don't! You already have a visa and you're only there to "validate, extend" something of that sort. Go to the entrance of the building and they should let you in immediately. The entire process only took about 20 minutes.

While I studied in spain this summer...I registered with the american embassy in Madrid, as well as the local police in Bilbao. I remember that very very long line...of course I waited in it only to be told that I had wasted my time ! Live and learn I guess...Keep the great advice coming!!