If you are standing in a public area, you can film whatever you want. Just dont literally stand in their way while they are trying to work, its not just illegal, its rude.
Some cops hate being filmed, I personally dont care so long as you keep a safe distance and allow me to do my job.
If you are on federal property though there is a law against any kind of recording and picture taking, federal cops WILL cite or book you on it so..... fair warning.
I think the federal policy will be challenged and changed.

