How to do people detection from bird eye view?
It appears that motion is your only cue in this case, assuming that you have a time-series of these images. I would start by trying to track these objects using a Kalman filter with a constant velocity motion model. The fountain and the street sign would be stationary. If your images contain other moving objects, e. g. cars, then you can try using speed to distinguish them from people.

Here is an example of how to track multiple objects using the Computer Vision System Toolbox for MATLAB. The example detects moving foreground objects using the Gaussian mixture models. You may want to replace that part or augment it using the depth information.