This is part of the festival. It’s called Correfoc, which is Catalán for Run of Fire or Fire Run or something like that. These are the demons being driven back to hell the day before the actual feast day.
I, however, thought this was going to be Sunday instead of Saturday and totally missed it. But you should watch them anyway because they are crazy. Those sparks will actually burn you, so you have to get out of the way. Like I said before, this city is so awesome. I can’t believe I missed this.