We present a methodology to estimate the number of attendees to events
happening in the city from cellular network data. In this work we used
anonymized Call Detail Records (CDRs) comprising data on where and when users
access the cellular network. Our approach is based on two key ideas: (1) we
identify the network cells associated to the event location. (2) We verify the
attendance of each user, as a measure of whether (s)he generates CDRs during
the event, but not during other times. We evaluate our approach to estimate the
number of attendees to a number of events ranging from football matches in
stadiums to concerts and festivals in open squares. Comparing our results with
the best groundtruth data available, our estimates provide a median error of
less than 15% of the actual number of attendees
