Born in India 71 years ago, Mohammed Rehat Ali is still traumatised a month after his release from a detention camp, struggling to shake off a fear for the future shared by millions -- many of them Muslims -- under Prime Minister Narendra Modi.

The push to render stateless people described as "infiltrators" by Modi's right-hand man has been limited to the north-eastern state of Assam, but his Hindu nationalist party wants to replicate it nationwide, alarming Muslims, who critics say are the real focus.

"I have never expected that I would have to prove my citizenship. I am an Indian citizen, we are born here in Assam and living here for generations," Ali, an illiterate farmer, told AFP. But when he was unable to produce the required documents, a "Foreigners' Tribunal" declared him a Bangladeshi and sent him to a detention camp. After three years, his sons secured his release by appealing to a higher court, but only after selling their land and cattle to raise legal fees.

He is one of the lucky ones. Over four million others in the state of 33 million -- where  immigration has been a hot topic since British colonial rule -- were left off a draft "National Register of Citizens" (NRC) last July.