Run the program like this:

python webcam.py

alpython.com/blog/python/face-detection-in-python-using-a-webcam/

Update: Now supports OpenCV3. This change has been made by furetosan ( https://github.com/furetosan) and tested on Linux.

To run the OpenCV3 version, run python webcam_cv3.py haarcascade_frontalface_default.xml