I'm a little confused. Is this party a one-day thing? Or will people have to travel and plan to be there for days? If it's a one-day thing, then telling people "No Pets Please" is not at all improper. Most people can leave their pets at home for a day.
If it's something where people have to stay longer then you also have to realize that they may not be able to come if they don't have a place or someone to take care of their pets. So, on this note, if you want them to come and know they won't be able to if they can't bring their pets, then it might be wise to indicate that pets will have to stay outside.
I suggest saying "No Pets Please" on the invitations. And then for those few exceptions that you don't mind if their pets come, call them up and explain the situation. I'm sure those few are going to understand.