India is preparing for a once-in-a-generation redraw of its political map, driven by what the government says is a push to reserve one-third of seats in parliament and state assemblies for women.

To do that, the government will unveil a constitutional amendment requiring a two-thirds majority, backed by a three-day special session starting Thursday.

Women make up only about 14% of India's 543 members of the lower house. The reform would raise that to roughly a third, closer to global norms.

Its rollout is now tied to a population-based redraw of constituencies based on the 2011 census - likely expanding the lower house from 543 to about 850 seats.

But this has fuelled controversy, with opposition parties accusing the government of rushing changes during an election season.

India already reserves 33% seats for women in village councils and municipal corporations in urban areas.

Prime Minister Narendra Modi has cast the Nari Shakti Vandan Adhiniyam (which loosely translates from Hindi to Saluting Women Power Act) as a historic leap - "among the most significant decisions of our times" - arguing it honours women's empowerment.

He said parliament was poised to take one of the century's "most important decisions", noting the quota had unanimous backing and was meant to be implemented by 2029.

Opposition parties say a simple women's quota is being tied to a controversial redraw of constituencies, turning a gender reform into a wider political overhaul.

"We support reserving 33% seats for women based on the current strength of parliament. We also want a further freeze on the expansion of overall parliamentary seats until population trends stabilise. We object to the haste in convening this session in the middle of an election season," John Brittas, MP of the opposition Communist Party of India (Marxist), told the BBC.

India's Constitution mandates that seats be allocated to each state based on its population, with constituencies of roughly equal size.

It requires periodic delimitation - the redrawing and reallocation of seats - after each census to reflect updated population figures.

So far, India has redrawn parliamentary seats three times based on the decennial census in 1951, 1961 and 1971. Since then, governments of all stripes have paused the exercise, fearing an imbalance of representation due to varying fertility rates across states.

The Modi-led Bharatiya Janata Party (BJP) government has signalled a break from past caution, proposing a fresh delimitation based on the 2011 census.

That shift has alarmed opposition parties - especially in the south - who fear it could cost them seats and influence, effectively penalising regions with lower population growth and stronger economies.

The five southern states - Tamil Nadu, Andhra Pradesh, Karnataka, Kerala and Telangana - account for about 20% of India's 1.4 billion people.

They also outperform the rest of the country in health, education and economic prospects. A child is less likely to be born here than in the north, due to lower population growth rates.

Their leaders are worried that the more prosperous south may lose parliamentary seats in the future, a "punishment" for having fewer children and generating more wealth.

Tamil Nadu, Kerala, Karnataka and Telangana had sought to extend the freeze on seat redistribution for 25 years.

Tamil Nadu Chief Minister MK Stalin has called the delimitation plan a "massive historic injustice," with his ruling DMK party calling statewide black-flag protests on Thursday.

"Is punishment being meted out to Tamil Nadu and the southern states for the crime of striving for India's growth?" he asked.

The proposed legislation also remains mired in some confusion, say experts.

Arghya Sengupta of Vidhi Centre for Legal Policy, a Delhi-based think tank, told the BBC that although the new bill raises the cap on the lower house of parliament to 850 seats - from 550 earlier - the "basis for this number is unclear and does not appear to be proportionate to population growth recorded in the 1971 and 2011 census".

Oddly, Sengupta says, there is no similar expansion for state assemblies, "creating a mismatch: fast-growing states could gain more MPs without a corresponding rise in MLAs".

There are also concerns about how seats will be shared between states.

BJP MP K Laxman has told reporters the government plans to carry out delimitation on a "pro-rata" basis to ensure no state - especially in the south - is disadvantaged.

This means each state's seats would rise in proportion to its current share.

But experts remain unconvinced.

"While no state may lose seats, the absence of a clear proportional formula means outcomes could vary - and favour more populous states. This has significant federal implications," says Sengupta.

It is also unclear how seats will be chosen for women in the expanded parliament.

"What criteria will be used to reserve seats for women? That's the tricky part. How do you decide which constituencies are set aside?" Sanjay Kumar of the Delhi-based Centre for the Study of Developing Societies told the BBC.

"It can't simply be based on population - and identifying seats for disadvantaged scheduled caste and scheduled tribe (SC/ST) women adds another layer of complexity," he added.

In India, constituencies are already reserved for Scheduled Castes (SC) and Scheduled Tribes (ST) based on their population share.

Critics also question why the exercise is based on 2011 data - asking why rely on 15-year-old figures when a fresh census is due soon?

The government counters that waiting for new census data would push delimitation - and with it women's reservation - well beyond 2029, delaying a long-promised reform.