President Pratibha Devisingh Patil was the first of the estimated 1.17 billion Indians to be counted at her pink sandstone presidential palace. Next, Congress Party chief Sonia Gandhi sat with the census-takers under a banyan tree in her yard while inspecting and signing off on a 29-point questionnaire listing income, religion, education and access to basic utilities among other topics.
The details may help authorities identify areas where neglect is high, where poverty is particularly rife and where high numbers of people are unable to read or to work.
"It is the second-largest census in the world" after China, Vice President Hamid Ansari told Press Trust of India news agency after being counted.
India could soon rival China as the world's most populous country, however, with a yearly growth rate of about 1.4 percent, while China - with about 1.3 billion people - is growing at a much lower 0.49 percent, according to 2010 estimates listed by the CIA World Fact Book.
Indian officials say the national count, taken every 10 years, is crucial for both the government and private sector to set relevant policies, programs and budgets. This is the second phase of the census, with the first last year listing some 300 million Indian households and estimating the population to be around 1.17 billion.
For the first time, the census is noting whether people live in mud huts or concrete structures, have electricity or access to toilets, and if they have ever been to school even if they don't go anymore.
Almost all residents, regardless of nationality, are included in the count, even those imprisoned like Pakistani Ajmal Kasab who is sentenced to death for his role in the 2008 Mumbai terror attacks.
The millions of homeless sleeping on railway platforms, under bridges and in parks will be last counted on the evening of Feb. 28, with revisions conducted until March 5 and the final census reports published over the next two years.
India has appointed some 2.7 million census-takers, many of them school teachers, who are also collecting name and address details to be used later in issuing identity cards as part of a National Population Register, for which citizens will also have to provide fingerprints and be photographed at some later date.
Officials warn that people trying to hide facts or give false answers may be fined up to 1,000 rupees (about $22).
India will conduct a separate caste census later this year, after officials decided the caste question was too controversial in the secular and multicultural democracy and could upset the results of the population count.