The arrested men told police the bones were sold to medical students and used in traditional medicine, district police chief Peeyush Pandey said.

Investigators found dozens of skeletons — treated with chemicals — laid out to dry in the sun when they reached a river bank in Keshia, 200 km (125 miles) north of Kolkata, capital of West Bengal state.

"We received complaints that several bodies were missing from graveyards and, while investigating the case, we stumbled on the secret bones factory," Pandey said.

The accused would take largely unburnt bodies from Hindu cremation sites as well as from rivers where the dead are often disposed by the poor who cannot afford to cremate them, he said.

The men were also suspected to have dug up Muslim graves to get bones which could fetch them thousands of rupees in the black market.

Trade in human bones and skeletons is banned in India and medical students are asked to study bone structure with skeletons made of fiber or plastic.