Water supply is off and toilet tank is empty. The bowl keeps filling up with water and is overflowing. Where is this water coming from?
You need to be more specific, is the water overflowing the bowl and running onto the floor? If that is the case then the drain line somewhere below that toilet is plugged and waste water from above the toilet that drains into the same sewer line is the problem. You will need to call a plumber who can figure out where the blockage is and fix the problem
Open the cap or cover on your sewer cleanout in the yard! If water/sewage comes out, then call the city emergency line immediately. Leave this cap off and let sewage flow into the yard. This will prevent it from overflowing into your house.