Well, it's a folk music site, but one of the most widely heard protest songs of the whole era was "War" as sung by Edwin Starr.

And the most sadly ignored was this gem.

Stevie Wonder had a few -- "Front Line" where he says, "They had me standing on the front line, now I stand at the back of the line when it comes to getting ahead." and "Living For The City" where he decries that the government won't give enough money to people of the inner city.