How can I remove a crowd of people from a scene while keeping one person in the shot
Is the camera moving? If it's a still shot you could make a clean plate of the empty city in Photoshop and use a green screen for the actor. If you don't want the shot locked-off you can cheat add some movement to the frame in post, but big changes in perspective would require recreating the city in 3D. 



Interestingly one of the earliest photographs of a human being was by accident — owing to the extremely long exposures required by the early emulsions, anything moving would blur out. In a photo by Louis Daguerre there was a guy standing having his shoes shined and he appears to be the only person in Paris. You could use the long exposure technique to get your background.