|The census showed a continued preference for boys over girls, with 914 girls for every 1,000 boys [GALLO/GETTY]
India's population has increased to 1.2 billion people this year, though growth actually slowed for the first time in 90 years, census officials have said.
The South Asian nation, second only to China in terms of its total population, added 181 million people in the past decade, said C Chandramouli, the census commissioner.
That increase alone is nearly the entire population of Brazil, and UN projections show that India could overtake China and its 1.34 billion people as the world's most populous nation by 2030.
Chandramouli said a more rigorous analysis of data would be needed before India made its own projections.
India's population is now nearly equal to the combined populations of the US, Indonesia, Brazil, Japan, Bangladesh and Pakistan, he said.
The numbers released on Thursday are preliminary, with official figures and analysis not expected to be released until next year.
The census, India's 15th since 1872, was a mammoth effort spread out over a year.
It involved 2.7 million census-takers who surveyed some 300 million households, noting for the first time whether people lived in basic huts or concrete structures, had electricity and access to toilets and if they had spent any time in schools.
The questions will help administrators develop policies and set budgets for a nation where 800 million people live in poverty.
Almost all residents, regardless of nationality, are included in the count. Millions of homeless were also counted.
The initial numbers show a decline in the number of children under the age of six, down five million since 2001 to 158.8 million.
The census also indicated a continuing preference for boys over girls in a country where female infanticide is still common and where the government has banned doctors from revealing the sex of unborn children.
A gender breakdown among children showed fewer girls than boys are being born or surviving, with 914 girls for every 1,000 boys under the age of six, compared to 927 for every 1,000 in the last census.
"This is a matter of grave concern," Chandramouli said.
Indians continue to favour sons over daughters mostly because of the enormous expenses involved in marrying off girls.
Even the poorest families are often likely to go into debt arranging marriages and paying elaborate
dowries to their daughter's new family.
"Whatever measures that have been put in over the last 40 years has not had any impact on child sex ratio and therefore that requires a complete review," GK Pillai, India's interior minister, said.
The overall gender-ratio showed a marginal improvement, with 940 women counted for every 1,000 men compared to 933 in the last census.
The census also showed that the literacy rate went up to 74 per cent nationwide for people aged seven and older, from about 65 per cent in the last count.