Two tribal leaders who are campaigning against the construction of a controversial mine in the eastern Indian state of Orissa were abducted and interrogated by plain-clothes police officers in what activists say is a significant upsurge in the intimidation campaign aimed at opponents of the mine.

Lado and Sana Sikaka, two senior leaders of the Dongria Kondh tribe, say they were kidnapped by 15 armed men on Monday morning as they left their villages in the Niyamgiri Hills.

They were on their way to Delhi to attend a conference organised by a coalition of environmentalists and tribal rights campaigners who claim the mine will cause an environmental catastrophe and force thousands of Kondh — one of India’s few remaining indigenous tribes — off their land.

The Orissa Mining Corporation, a subsidiary of the London-listed industrial giant Vedanta Resources, is trying to build a bauxite mine deep within the Niyamgiri Hills to provide raw material for an alumina refinery that has already been constructed at the base of the hills.

The 125bn rupee (£1.7bn) project has emerged as a test case in India pitting industrial interests against those of indigenous people and the environment.

Vedanta says the mine will bring much-needed investment into a remote part of India. The company says the Kondh, who worship the hills as their god and forage in the forest for food, will be adequately catered for. But environmentalists and tribal leaders reject those claims and have led a vociferous protest movement to oppose the mine over the past three years.

Tensions in the remote area have risen recently, and the Indian government is due to make a decision in the next few months on whether construction can go ahead.

Local activists and journalists said Sana Sikaka was released by his captors after 12 hours. But Lado, the Kondh’s most senior tribal leader, says he was held for two nights in a police station where he was interrogated and beaten by police officers.

Surya Shankar Dash, a film-maker based in Orissa’s state capital Bhubaneswar, told The Independent he had spoken to Lado Sikaka by mobile phone yesterday afternoon.

“He was beaten up, interrogated and made to sign some papers,” Mr Dash said. “To begin with he was not sure who had abducted him. The men wore plain clothes, but were heavily armed with sophisticated weaponry. It was only later, when they took him to a police station, that he knew they were police.”

It is believed that Lado was held by police at Rayagada, a district town to the south of the Niyamgiri close to the border with Andhra Pradesh. As Lado cannot read or write, it is not known what papers he signed.

Siddharth Nayak, a tribal rights campaigner, was with Lado and Sana when they were abducted on Monday. He says two SUVs filled with armed men stopped their convoy as it was leaving the Niyamgiri Hills on the road towards a local airport, where the group was planning to board a flight to Delhi for yesterday’s conference.

“The men kept asking why we were opposed to the Vedanta, why we were opposed to the mining,” he said.

Witnesses say that Lado, who was interviewed last week by Channel 4 News about his opposition to the mine, was dropped off by police yesterday shortly after midday local time in Kalyansingpur, 10km away from his village of Lakhpadar.

Amnesty International has called on the Indian government to investigate the kidnappings, stating that Orissa’s provincial police refused to open an investigation into the disappearance of the two men.

Madhu Malhotra, Amnesty’s deputy-director for the Asia-Pacific region, said: “This allegation of arbitrary detention and abduction of activists must be immediately and transparently investigated. The Orissa police must show its good faith by immediately tracking down and arresting these gunmen.”

The Dongria Kondh have previously accused police and armed thugs of intimidating and harassing them. But local activists say the kidnapping of two such high profile figures is a profound shift in tactics.

Bratindi Jena, head of tribal rights at Action Aid India, regularly travels to the Niyamgiri region and is one of the few outsiders who is fluent in the Kondh’s language.

“This is quite an unusual event and we need to find out exactly what has happened,” she said. “People have been arrested before, we have had people beaten before but they don’t normally abduct tribal leaders.”

Stephen Corry, director of Survival International, added: “It is abhorrent that in the world’s largest democracy, a man who speaks out in defence of his land and his community should be ‘disappeared’ in this way.”

The mining controversy

Vedanta’s planned bauxite mine in Orissa has brought the company a great deal of controversy in the past, and it will be dismayed to have the project again associated with such negative headlines. In February, the Church of England was sufficiently concerned about the proposed mine to sell all of its shares in Vedanta, explaining its decision by questioning “the level of respect for human rights and local communities”. The church was not the only investor to decide that it did not want to be associated with the Orissa mine. The Norwegian government sold a $13m (£8.3m) stake in 2007, and BP’s pension fund followed suit.

Those moves, coupled with the condemnation of celebrities including Michael Palin and Bianca Jagger, have made the company’s brand one of the most vexed in business. Vedanta’s chief executive, M S Mehta, has dismissed the allegations against it as “lies and hoax”, insisting that the operation would be good for the people in the forests near the planned sites. In general, though, Vedanta has often been tight-lipped about its record. Until a protest against the Orissa development earlier this year, it refused to engage with pressure group Survival International over its plans, and repeatedly rejected interview requests, a strategy which it ultimately admitted was “naive”.

Anil Agarwal, the company’s majority owner, has not been much more forthcoming on the issues that blight his firm. One of India’s richest men, with an estimated personal wealth of $6.4bn, the so-called “Bollygarch” made his first fortune in scrap metal. When he does speak, he insists his motivations are philanthropic: he points out that he has promised to spend $1bn to build a university in Orissa. But his company’s critics will be hard to convince.