Stay away from it. We've had several attacks here lately and unfortunately some of them have ended in loss of life.
DO NOT RUN if it's outdoors and sees you.
Actually, check the local ordinances in your town/village/city... There may be a leash law. If there is, and the dog is loose, then call the authorities and have it picked up.
Just be very cautious. They are quite strong and surprisingly fast.
Sorry to barge in on the Teen board, but I've been attacked and bitten badly by dogs, so it's a very sensitive topic with me!