Millions of census workers across India today began a mammoth effort to document more than a billion people in the world's second most populous country.

First counted in New Delhi was President Pratibha Devisingh Patil at her presidential palace.

Next, Congress Party chief Sonia Gandhi sat with the census-takers under a banyan tree in her garden while filling in a 29-point questionnaire listing income, religion, education and access to basic utilities among other topics.

The details may help 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," Vice President Hamid Ansari said after being counted.

India could soon rival China as the world's most populous with a yearly growth rate of about 1.4%, while China - with about 1.3 billion people - is growing at a much lower 0.49%.

India says the census, taken every 10 years, is crucial for both the government and private sector to set policies, programmes 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 do not 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 February 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 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 £13.50).

India will conduct a separate caste census later this year, after officials decided the question was too controversial in the secular and multicultural democracy and could upset the results of the population count.