Any government in budget-cutting mode is going to hear howls from the folks on the sharp end of the knife. It's going to be doubly hard for a conservative party to be the one to do the cutting, since the left is going to be against them even if they were spending like a sailor.
That's where the Spanish government is at, having to cut a deficit in the middle of a recession; given the lousy European economy and jittery markets, running a big deficit just isn't feasable without paying Lenny the Loan Shark interest rates. The conservative Popular Party took over late last year and got handed one of Europe's worst budget deficits from their Socialist predecessors.
Of course, you're going to see protests. What else would a red-blodded leftist do on a Saturday afternoon (well, go see Barcelona mop the floor against Rayo Vallecano, but if you don't have tickets...) other than go to a protest?
Speaking of Barcelona, the Catalan area of Spain is better off, so it net-to-net sends money to the poorer parts of the country; it's been itching for more autonomy from Madrid and such. So, in those areas that don't like the central government much to begin with, it's easy to take to the street.
Good luck, PM Rajoy; you'll need it.