A while back, when Obama had his vacation in Spain, there was a bit of a kerfuffle about the fact that the American consulate in Barcelona was warning African Americans that the Barcelona police was racist and had arrested and injured an unnamed black person without reason. The warning was quickly scrubbed and the incident forgotten, until Wikileaks came along to tell the real story.
In early 2009, a crew member from a US naval vessel was arrested for assaulting a police officer on rather flimsy grounds. The crew member had thought that the plainclothes officers were trying to rob him, and they alledged that he may have been trying to buy drugs. While in police custody, the crew member was knocked to the ground and needed several stitches. Although there was some kind of internal investigation, the US was very unhappy with the lack of real results and I imagine the warning came not to soon after.
I think there are different standards in Spain as to what consitutues police brutality. All this stuff you heard on American TV about the right to remain silent, etc, don´t expect to get it in Spain. From what I´ve heard, they get physical in interrogations pretty quickly and none of the lawyer or phone call nonsense.