Since arriving in Spain, I have seen four organized groups of demonstrators. They often appear downtown, slow or stop traffic, and are often accompanied by musical instruments. Seriously, everything from a drum to a marching band. So, when I wasin a bit of a hurry to meet someone at a café, it was a little unnerving to run into a protest that effectively blocked every street in the direction that I was traveling. Conveniently, I had my camera and could turn it into a bit of a tourist moment.
On a more societal note, the act of demonstrating is considered one of the key rights of a Spanish person, and I have seen a wide variety of people involved in the protests. I have no idea whether they are effective or not, but they are certainly interesting to watch, since they are, for the most part, pretty peaceful. For the most part, police simply held back the traffic so the protest could stay peaceful, too.