I guy I work with was just arrested for taking pictures of cops restraining his step kids. The kids don't want to see their real father and the cops came to pick them up.
He stood on his front porch, he didn't want to get any closer for fear of giving them an excuse to shoot him. He had a baby in one arm and a camera in the other. He was arrested and locked up for 24 hours. They didn't charge him until the next week when he called the police chief and wanted to know why he was jailed. When he said the magic word "sue" then he got charged. He was told when his court date was, but not the charges. He finally found out what the charges were when he went down to the court and talked to a few different people and made it known that he wouldn't leave until they told him what he was being charged with. They are charging him with something they call "obstructing government business" and have promised he'll do 6 months for it and pay a $500 fine.
He's talked to a few lawyers and they tell him that what the cops did is illegal, and there's almost no way he can win. He'll have to go to court, he'll lose and then do his 6 months while he appeals. A couple of the lawyers even told him that's it's common knowledge that no citizen ever wins in their court.
So anyway, there doesn't need to be any law against it for you to go to be arrested and go to jail for it. At the very least, you should retain a lawyer and count on going to jail before photographing any cop.