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 for women in parliament and state assemblies.

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

Women make up only about 14% of India's 543 upper house MPs. 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.