Some 2.7 million officials fanned out across the country on Wednesday to begin the mammoth exercise of headcounting India's estimated 120 crore (1.2 billion) people, with President Pratibha Patil being the first citizen to be enumerated.
The nationwide exercise, which will continue over the next three weeks, began as a group of officials, called enumerators, visited the president at the Rashtrapati Bhavan.
Census enumerator Rita collected information on a number of characteristics about the president's household.
Pratibha Patil signed off on a 29-point questionnaire listing income, religion, education, access to basic utilities and other topics. A similar form will be filled by every citizen of the country through February 28.
Home Minister P Chidambaram, Minister of State for Home Affairs Gurudas Kamat, Census Commissioner C Chandramouli and Director Census Operation (Delhi) Varsha Joshi were present at the Rashtrapati Bhavan.
Speaking after her enumeration, the president asked everyone to "wholeheartedly" take part in the census, considering it "as a national duty".
Vice President Hamid Ansari, Prime Minister Manmohan Singh and Congress party chief Sonia Gandhi also provided the details about their respective households to the enumerators.
The family members of governors and chief ministers were among those enumerated on the first day of the second phase.
A similar exercise would be carried out in more than 7,000 towns and 600,000 villages across the country, including in troubled Jammu and Kashmir, northeast and Maoist insurgency-affected areas.
According to officials, around 100,000 enumerators and supervisors have been appointed in the eight northeastern states. Soldiers and paramilitary troopers have been engaged as enumerators in their respective camps and barracks.
In Jammu and Kashmir, even separatist leaders have supported the exercise, urging the people to provide full details to census officials. The first phase of the exercise was held from July 15 to Sep 30 last year, which passed off peacefully. The state had missed on the exercise in 1991 when insurgency was at its peak.
In Orissa, the population count began with the enumeration of Chief Minister Naveen Patnaik. The chief minister also flagged off a publicity van, which will travel through various parts of the state during the second phase that will conclude Feb 28, an official said.
The first round of houselisting and collecting housing data for Census-2011 was completed from April to September last year.
The houselisting is done because there is no complete address system in India and each and every structure is being listed so that when the enumerators come for headcounting, they know exactly where to go, officials said.
In the second phase, enumerators will ask questions on literacy, work status, marital status, languages spoken, mode of transport and number of children.
On the last day of the survey, enumerators will count the homeless on the streets across India. And between March 1 and 5, they will compile the data for the entire population.
The gigantic decadal exercise, which will be the 15th headcount of India's population since 1872, is undertaken to create a database on demography, economic activity, literacy and education, housing and household amenities, urbanisation, fertility and mortality, Scheduled Castes and Scheduled Tribes, language, religion and migration.
New features added to the 2011 census include revised questions on the institutional household, new category in gender parameter for transgenders, a code for separated and divorced, new codes under status of school attendance and a separate code included under non-economic activity.
Census 2011 has also introduced new initiatives to sensitise school students about census operations.
For the first time, officials are stepping beyond the demographics and economic activity and would collect details like ownership of mobile phones, computers, internet access, and availability of treated or untreated drinking water.
The census is the only source of credible data base in India that the government uses to formulate its policies.