If the dogs are not supposed to be off leash, then I would contact the city (b/c the police wouldn't do much where I live in Vancouver BC). There is a fine of up to $2,000 in my city if you're violating a by-law such as having a dog off leash in a LEASHED area.
Are there any signs up in the park w/warnings of fines and a number for you to contact? If not, I would definately look up the by-laws that are enforced in your area...then you can give the city this information.
I hope that helps
