This question already has an answer here:
In the Dark Knight, Bruce Wayne tracks Melvin White
aggravated assault, moved to Arkham twice
to his apartment, which is overlooking Commissioner Loeb's funeral parade, where he finds a police honor guard unit tied to a post, blindfolded, mouths taped shut. When Bruce removes the tape from one of the men, Bruce asks:
They took our guns, our uniforms
From this we can assume that a substantial portion, if not the entire unit, is full of the Joker's men. Why doesn't anyone realize this? Surely there must be someone familiar with the individuals in the honor guard.
Previous answers have stated that a corrupt police force allowed the Joker and his men to blend in, or that there were so many police in uniform that all looked the same, or that Gordon and his men were too busy scanning windows. Still, none of these explanations account for the fact that the honor guard was still a separate part of the procession and had a special role. There is next to no chance they could completely escape detection.