I have a city, and let's say this city gets cut off, Berlin '45 style. Nothing in, nothing out. Classic siege. I want to be more specific than that though: Suburbs won't be in it, it will be all the most densely populated parts of a modern city, like Munich or London. No prep time, just suddenly as if a dome were put over it. No Rosinenbomber flying in food or any other input. No one can also leave or enter. While I want to know both with and without electricity/running water, if you can only answer one I'd like to know without either more.
Of course a city can't sustain itself for long, but how would the population curve look like? How quickly until the population has halved? How many people could, in a city like Munich (~1M people), actually live/where would the population level out? The people could fish in rivers that flow through the city, but if it has a coast they could not swim out much. (I know this still leaves a lot of variables, but I am still not sure how the city will actually end up. You can really take any western modern city as example for this for I can translate it to what I will need)