Across a river in a remote part of India's northeast, labourers have cleared dense forest in an area equivalent to about seven soccer fields and are building the first mass detention centre for illegal immigrants.

The camp in the lush, tea-growing state of Assam is intended for at least 3,000 detainees. It will also have a school, a hospital, a recreation area and quarters for security forces - as well as a high boundary wall and watchtowers, according to Reuters interviews with workers and contractors at the site and a review of copies of its layout plans.

Some of the workers building the camp said they were not on a citizenship list Assam released last week as part of a drive to detect illegal immigrants. That means the

workers could themselves end up in detention.

Shefali Hajong, a gaunt tribal woman from a nearby village, said she was not on the list and will join nearly two million people who need to prove they are Indian citizens by producing documents such as birth and land ownership certificates dating back decades.

If they fail to do so, they risk being taken to detention camps like the one being built. The government says there

are hundreds of thousands of illegal immigrants in Assam from neighbouring Muslim-majority Bangladesh, but Dhaka has refused to accept anyone declared an illegal immigrant in India.

Shefali, who belongs to the indigenous Hajong tribe, said she was tense because of the situation.

"But I need to fill my stomach," she said in the local Assamese dialect as she used a hoe to feed stones into a concrete mixer. She and other workers make about $4 a day, which is considered a decent wage in the impoverished area.

She said she didn't know her exact age and believed it was about 26, adding that she did not know why she wasn't on the citizenship list. "We don't have birth certificates," said her mother, Malati Hajong, also working at the site.

The camp, near the town of

Goalpara, is the first of at least ten detention centers Assam has planned, according to local media reports.

The mammoth Supreme Court-ordered exercise to document Assam's citizens has been strongly backed by Prime Minister Narendra Modi's Hindu nationalist government that came to power in New Delhi five years ago. Critics say the campaign is aimed at Muslims, even those who have lived legally in India for decades.