Cops who act illegally should be fired and prosecuted, every time. They hold a position of public trust and need to be held to a higher standard than civilians, not a lower one.
UPDATE: Police claim protesters were carrying rocks. They arrested about 100 protesters. The group Recreate '68 says it was denied its use of a legal permit for the use of Civic Center Park, while police helped to protect and bring in Rev. Fred Phelps of the Westboro Baptist Church. About a dozen abortion protesters were arrested on Tuesday, so they weren't being given special treatment.
In the Denver Post's photos, I don't see any cops without visible badges, though in only a few photos of cops with riot gear are the pictures close enough to see the numbers in white on the front of their uniforms.