India has published the final version of a list that effectively strips about 1
The National Register of Citizens (NRC) is a list of people who can prove They came to the state by 24 March 1971, a day before neighboring Bangladesh declared independence from Pakistan.
People left off the list would have 120 days to appeal against their exclusion.
It is unclear what happens next.
India says the process is needed to identify illegal Bangladeshi migrants.
It has already detained thousands of people suspended from being foreigners in temporary camps that are housed in the state's prisons, but deportation is currently not an option for the country.  The process has also sparked criticism of "witch hunts" against Assam's ethnic minorities.
A draft version of the list published last year had four million people excluded.
What is the registry of citizens?
The NRC was created in 1951 to determine who was born in Assam and is therefore Indian, and
The register has been updated for the first time.
Families in the state have been required to provide documentation to show their lineage, with those who cannot check their citizenship against illegal foreigners.
Prime Minister Narendra Modi's Hindu nationalist Bharatiya Janata Party (BJP) has long railed against illegal immigration in India but has made the NRC a priority in recent years.
Why is registry happening in Assam?
Assam is one of India's most multi-ethnic states. Issues of identity and citizenship have long vexed a vast number of people living there.
Among its residents are Bengali and Assamese-speaking Hindus, as well as a medley of tribespeople.
A third of the state's 32 million residents are Muslims, the second-highest number after Indian-administered Kashmir. Many of them are descendants of immigrants who settled there under British rule.
But illegal migration from the neighboring Bangladesh, which shares a 4,000-km-long border with India, has been a concern there for decades now. The government said in 2016 that an estimated 20 million illegal immigrants were living in India.
Have 1.9 million people effectively become stateless?
Not quite. Residents excluded from the list can appeal to the specially-formed courts called the Foreigners Tribunals, as well as the high court and the Supreme Court.
However, a potentially long and exhaustive appeals process will mean that India's already overburdened courts will be further clogged, and poor people left out of the list will struggle to raise money to fight their cases.
If people lose their appeals in higher courts, they could be detained indefinitely.
Some 1,000 people declared as foreigners earlier are already lodged in six detention centers located in prisons. Mr Modi's government is also building an exclusive detention center, which can hold 3,000 detainees.
"People whose names are not on the final list are really anxious about what lies ahead. One of the reasons is that the Foreigners Tribunal does not have a good reputation, and many people are worried that they will have to go through this process, "Sangeeta Barooah Pisharoty, author of Assam: The Accord, The Discord, told the BBC.
Why have the courts been controversial?
The special courts were first set up in 1964, and since then they have declared more than 100,000 people foreigners. They regularly identify "doubtful voters" or "illegal infiltrators" as foreigners to be deported.
But the workings of specially formed Foreigners Tribunals, which have been hearing the contested cases, have been mired in controversy.
There are more than 200 such courts in Assam today, and their numbers are expected to go up to 1,000 by October. (The majority of these tribunals have been set up after the ruling Hindu nationalist BJP came to power in 2014.)
The courts were accused of bias and their workings were often opaque and riddled with inconsistencies.
For one, the burden of proof is on the accused or the alleged foreigner.
Second, many families are unable to produce documents due to poor record-keeping, illiteracy, or because they lack the money to file a legal claim.
People have been declared as foreigners by the courts because of differences in spellings of names or ages in voters rolls, and problems in obtaining identity documents certified by authorities. Amnesty International has described the work of special courts as "shoddy and lackadaisical."
Journalist Rohini Mohan analyzed more than 500 judgments from these courts in one district and found 82% of people on trial had been declared foreigners. She also found more Muslims had declared foreigners, and 78% of orders were delivered without the accused having ever heard – the police said they were "absconding", but Ms Mohan found many of them living in their villages and unaware they were declared foreigners.
"The Foreigners Tribunal," she says, "must be made more transparent and accountable."
A decorated Indian Army veteran, Mohammed Sanaullah spent 11 days in a detention camp in June after being declared a "foreigner".
Both the citizen's register and the tribunals also have sparked fears of a witch hunt against Assam's ethnic minorities.
Many say the list has nothing to do with religion, but activists see it as targeting the state's Bengali community, and a large proportion of whom are Muslims.
They also point to the plight of Rohingya Muslims in neighboring Bangladesh.
However significant numbers of Bengali-speaking Hindus have also been left off the citizenship list, underscoring the communal and ethnic tensions in the state
"One of the communities worst affected by the list are the Bengali Hindus. There are as many of them in detention camps as muslims.This is also the reason just days before the NRC is published the BJP has changed tack, from taking credit for calling it an error-ridden.This is because the Bengali Hindus are a strong voter base of the BJP, "says Ms Barooah Pisharoty.
The human tragedy
Fearing possible loss of citizenship and detention after exclusion from the list, scores of Bengali Hindus and Muslims have killed themselves since the process to update the citizen registry started in 2015, activists say.
And in an echo of US President Donald Trump's policy on separate undocumented parents and children, families have been similarly broken up in Assam.
Detainees have complained of poor living conditions and overcrowding in the detention centers.
One detainee told a rights group after his release that he was taken to a room that had a capacity of 40 people, but was filled with about 120 people. People who have been declared foreigners as well as many inmates have been suffering from depression. Children have also been detained with their parents.
Human rights activist Harsh Mander who has visited two detention centers has spoken about the situation of "grave and extensive human distress and suffering."
What happens to people who are declared foreigners?
The BJP which rules the state, has insisted in the past that illegal Muslim immigrants will be deported. But neighborhood Bangladesh will definitely not accede to such a request.
Many believe that India will end up creating the newest cohort of stateless people, raising the specter of a homegrown crisis that will echo that of the Rohingya people who fled Myanmar for Bangladesh .
It is not clear whether people stripped of their Indian citizenship will be able to access welfare or property.
One possibility is that once they are released, they will be given work permits with some basic rights, but
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