Call Animal Services immediately and be prepared to give a detailed description of the dog. If you know where the dog lives give that address to the shelter staff. We will go directly to the pet owner and address the concern with them.
If the dog is friendly, put it in your yard and hold it until the officer can pick it up. Often times the dogs are gone by the time an officer can respond. If you feel threatened at all by the dog please do not risk getting bitten, call Animal Services and let them know about its aggressive behavior.