Nigerians have been carrying out their first census for 15 years, in what is a politically sensitive operation. Each Nigerian is thumb-printed after being counted.
After delays and protests, the authorities promised not to ask questions about religion and ethnicity. An area's political power and resources are influenced by its population.
Almost one million enumerators are being deployed across Africa's most populous country in the five-day process.
The normally congested streets in Lagos have been eerily quiet, as enumerators go house-to-house counting residents, who have been told to stay indoors.
Estimates suggest the population could be anywhere between 120m and 150m. The last count in 1991 found 88.9m.