India has published a list which effectively strips some four million people in the north-eastern state of Assam of their citizenship.
The National Register of Citizens (NRC) is a list of people who can prove they came to the state by 24 March 1971, when Bangladesh was created.
India says the process is to root out hordes of illegal Bangladeshi migrants.
But it has sparked fears of a witch hunt against ethnic minorities in Assam.
Fearing violence, officials say that no-one will face immediate deportation.
However, they say that a lengthy appeal process will be available to all - even if it means millions of families will live in limbo until they get a final decision on their legal status.
At present, that seems little more than a threat aimed at whipping up Hindu support for the BJP ahead of elections.
No deportation procedures have been put in place, and Bangladesh, already burdened by the Rohingya crisis, has shown no sign of being open to accepting a raft of new refugees.
Nonetheless, campaigners like Samujjal Bhattacharyya are clear that something must be done.