I mean maybe, it's possible. I just don't see it being likely.
First, because not getting caught would be very difficult. Be that by police, or in reality citizen journalists. If they can try to spin the police cleaning this shit away as them placing it there, I don't think you could get away with this on a large scale with so many camera phones around.
Second, if you get caught you're fucked - like completely and utterly fucked.
Given the above the question really is what is more likely:
These are bricks left out by construction workers or there was a great unseen conspiracy to get them into place.