The full results of India's national census will be released in mid-2011
India is launching a new census in which every person aged over 15 will be photographed and fingerprinted to create a biometric national database.
The government will then use the information to issue identity cards.
Officials will spend a year classifying India's population of around 1.2 billion people according to gender, religion, occupation and education.
The exercise, conducted every 10 years, faces big challenges, not least India's vast area and diversity of cultures.
Census officials must also contend with high levels of illiteracy and millions of homeless people - as well as insurgencies by Maoists and other rebels which have left large parts of the country unsafe.
President Pratibha Patil was the first person to be listed, and appealed to fellow Indians to follow her example "for the good of the nation".
"Everyone must participate and make it successful," she said in Delhi.
This is India's 15th census and the first time a biometric element has been included.
Current population: 1,139,965,000 (2008)
Fifteenth national census since 1872
"House listing" begins on 1 April
Physical count of residents takes place from 9 to 28 February 2011
Every person above the age of 15 to be photographed, fingerprinted and given 16-digit identity number; new National Population Register
Full census results in mid-2011