It is my personal opinion but, I think you won't have any problem. The protest of the other day was totally peaceful, and though the public opinion is clearly against the war, I think that the protests will continue to be peaceful (when a huge amount of people in Spain protests even old people and small children and babies go to the protest, it would be stupid if these same people got violent).
Perhaps some people could be nasty with you (if that is the case, ignore them!). I think that the majority (>90%) of the people you will talk with will be nice and respectful (extremists are everywhere worldwide).
It is not a good idea either to publicly state your opinion for the war (if that is the case); though I have done it (not for, but neither totally against) without any problem (people is just somewhat excited with the topic).
Enjoy! And if you have any other question or need help planning the trip just post here
Fernando