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.
"India has been conducting a national census since 1872," the man leading the exercise C Chandramouli told the AFP news agency. "Nothing - floods, droughts, even wars - has been able to stop it.
"The trick is to get things right the first time. There is no question of a re-census."
Over the next year, some 2.5 million census officials will visit households in more than 7,000 towns and 600,000 villages.
The officials, many of them teachers and local officials, will first begin the process of house listing - which records information on homes.
This count will, for the first time, also attempt to gather information on the use of the internet and the availability of drinking water and toilets in households.
The physical count of residents will take place from 9-28 February 2011.
The mammoth registration exercise will stretch over 11 months, consume more than 11 million tonnes of paper, and cost 60bn rupees ($1.3bn; £880m).
India's Home Minister, P Chidambaram, has described the process as the biggest of its kind in human history.
"An exercise of this kind has not been attempted anywhere else in the world," he told reporters in the capital.
The national census is the only source of primary and credible data in India and is used not just to formulate government policies but also by private companies to identify markets for their products, says the BBC's Sanjoy Majumder in Delhi.
The first 16-digit identity numbers are due to be issued starting in November.
The full census results will be released in mid-2011.
The collation of biometric information on a national database raises questions about possible infringement of civil liberties in the future.
BBC correspondents say many Indians support the new ID cards, believing they will make it easier to receive help and benefits from the state and remove the current need for multiple personal documents.
I think it is good that we are creating the national database of all our citizens. This will help maintain law and order, minimise crimes and help in locating people responsible for crimes. This will also ensure government benefits reach everybody and we will know who is left out. It will help individuals in getting house or land registrations, opening bank accounts and getting employment easier. These things usually take a lot of time because of background checks and the numerous documents required. I think this is a great job that the government is doing.Sandeep Singh, Bangalore, India
It's high time we kept up with technology. US citizens have a social security number. We also need a database of all citizens living in this country. I am sure it will be worth the time and money spent. Javs, Mumbai, India
I feel very uncomfortable about this fingerprinting and face-photographing exercise. Tomorrow they may ask each one of us to have a chip, so that "they" can always know where we are. Whatever happened to freedom and liberty? But given the terror attacks that have already happened in India we, the sheep, will willingly undergo this national haircut! Kailash, Chandigarh
It sounds like a good initiative as it will help people in remote and underprivileged areas to come in direct contact with government policies made for them - until now the government has been dependent on corrupt local bodies. The influx of people from villages to towns and from towns to big cities can be checked more efficiently and measures can be taken accordingly. But the data should be treated as extremely confidential and should not be used for profit-making activities. Faizan, New Delhi
Issuing unique ID numbers to all citizens as a part of biometric census is overdue. It will greatly improve tax collection. It will check corruption as well as loopholes in implementing social welfare measures in India. Due to these reasons all society conscious people are eagerly awaiting the results. In no way will the biometric national database infringe on people's civil rights. I think this project will itself bring the Congress Party into power again in next elections. Sambasiva Rao Gogineni, Vijayawada, India
I support the process. It is important. It will result in less corruption. It will benefit all law-abiding people. K.Venkataraman, New Delhi, India
Some infringement of rights notwithstanding, this exercise will go a long way in improving the internal security of the country. It will bring the entire 1.2 billion population of India under one database. Hats off, a mammoth exercise for a gigantic country. Wishing all the best to the officials conducting it! Anubhav Saxena, Mumbai India