Officer Tanya Little says it happened around 2 p.m. Thursday on Second Street, just a few blocks from the Liberty Bell and Independence Hall.
The pit bull went up to the horse and bit it on the upper left leg.
When the horse kicked the dog, the dog tried to bite him again. Eventually, the dog retreated and the horse was taken to the stable for treatment.
The Philadelphia Animal Welfare Society says the dog, named Kelly, is a rescue dog up for adoption and was being walked by a volunteer when she bit the horse.
The society says Kelly was rescued after someone threw her out of a moving car two months ago.
The SPCA says police now have the dog in custody and the dog will be held at the SPCA until police complete their investigation.
A message left with Philadelphia Trolley Works, which runs the horse-drawn carriage tours, wasn't immediately returned.
The horse was taken away from the scene in a trailer.
The Associated Press contributed to this report.
