Leaders of most major political parties in India, spurred by census results that show this already overpopulated country has 12 million more people than the experts predicted, are calling for a "vigorous" national birth control program.

A leading Indian industrialist, J.R. D. Tata, a supporter of family planning measures for more than 30 years, called this rare display of political unity evidence of "a consensus of the whole country on the threat, implications and consequences of our excessive population growth."

He said that the unchecked population growth has robbed India of the fruits of 30 years of economic development and played a major role in caste and religious violence that is increasing in the country.

India's Family Planning Association, a private organization of which Tata is chairman of the board, organized a declaration containing the call for birth control action. It was signed by 83 political leaders, industralists, academics and journalists. Among those who signed was the president of Prime Minister Indira Gandhi's Congress - I Party, as well as heads of all major opposition parties except the Communists. wtIt is unclear how this declaration will be translated into official policy by the government, which has soft-pedaled population programs since charges of forced sterilization in 1977 helped drive Gandhi from power for nearly three years.

The latest census figures released in mid-March are known to have shocked politicans and government planners. They showed that India, the world's second most populous country behind China, has 684 million people -- at least 12 million more than official projections had estimated.

Furthermore, India's growth rate remained almost the same in the 1970s as in the 1960s -- 24.75 per thousand -- despite $875 million spent on birth control programs in the last 10 years.

Thus most experts here believe India has no chance of reaching its goal of stabilizing its population at around 950 million by the turn of the century. If the population growth continues at the present rate, Tata said, India will have 1.5 billion residents in 50 years -- more than double the present population.

India's population has already doubled in the 34 years since independence. Census Commissioner Pedatala Padmanabha, even more pessimistic than Tata, estimated that under the present rate of growth it will take only 20 years for the population to double again.

Rami Chhabra, a journalist specializing in population problems, said the census figures "amount to a searing indictment of the administration of the family planning program."

In a speech last Wednesday to the World Health Organization in Geneva, Gandhi defended India's family planning program as having prevented 9 million births in the past decade and said some of the population increase is due to people living longer, not to a higher birth rate. Yet she acknowledged being "disturbed" by the "alarming increase" in India's population as shown by the census.

Tata said last Friday that his company's experience in providing family planning services to its employes has shown that Indians -- especially women -- clamor for birth control if it is made available to them at no cost.

About one-half of India's population lives under the poverty line, and Tata said "they will still live in abject povert" in 50 years, despite massive economic growth, if the country's population is not checked.