It totally depends on the cats personality, age of the cat, or age of the dog. If the cats a kitten, it's going to be afraid his and such, but not much will come of it since it's a kitten. If the cats older the cat will be more aggressive, but probably not in the manner of chasing the dog off, just hissing, backing up, swinging, etc. And the dogs going to matter on it's curiosity.

Now I'm not saying these are a given, but just usually how it is. But they adapt in not much time. And cats usually don't go and attack what they are afraid of, they will just keep their distance and hiss at things that scare them. So it won't be like some big cat fight (unless the cat don't take shit). This cats most likely socialized a lot, especially for it to be that comfortable on a leash.

Either way, had cats and dogs all my life, it's great. And cats and dogs can keep each other company too. My parents dog use to love to run the neighborhood. We had cats we he was little, but they died and after that we'd hear reports of him every wed and sunday down at the local church at the exact time they got out to soak up all the petting he could get. And we'd catch him going the other way on the road. Then we got a cat and he just stopped. We had tried putting him on a chain to go to the bathroom and only off the chain when we were around and he would still bolt right in front of us. Got the cat and he never ran, even though that cat was a bit crazy and not afraid of him at all. He just seemed much happier having something around when we were gone and something besides us.

So like I said, even if its a rocky introduction, they will come around.