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The chief of a banned upper-caste private army was shot dead by unknown gunmen in India's eastern state of Bihar on Friday, police said.

Brahmeshwar Singh headed the Ranvir Sena, a private militia of upper caste landlords allegedly responsible for several mass killings of low-caste people or Dalits.

Nicknamed the Butcher of Bihar, Singh faced charges in connection with killings in the 1990s that left 277 farmers and villagers in the province dead.

Singh, 67, was on a morning walk near his home in the province's Bhojpur district when he was shot dead by unidentified assailants.

“He died on the scene. Protesters prevented the police from taking the body for autopsy. The situation is extremely tense,” senior police official SK Bharadwaj told reporters.

State authorities deployed additional security forces in the area as Singh's supporters attacked government offices and set fire to vehicles in protest.

The society of rural Bihar has an almost feudal structure, characterised by widespread caste discrimination.

Singh formed Ranvir Sena in 1992, and fought with Dalit groups and Maoist insurgents on behalf of upper-caste landlords.

The group was accused of several massacres, including one in the Laxmanpur Bathe area in 1997, in which 61 Dalits including women and children were killed.

Singh was arrested in 2002 and imprisoned for nine years in connection with several cases including the Laxmanpur Bathe incident. He was released on bail last year.

Singh has said he was fighting for the rights of farmers and denied the charges against him, which include murder. - Sapa-dpa