Police in India's northeastern state of Bihar are on high alert after gunmen killed the head of Ranvir Sena, a right-wing, upper-caste militia group.

Brahmeshwar Singh, in his early 70s, was killed in the Bhojpur district while taking an morning walk, the Press Trust of India said.

He was confronted by six gunmen who shot him several times, The Indian Express newspaper said.

Violence broke out in the town of Ara after word of the killing spread and police at the shooting scene were heckled by Sena supporters, The Express said.

Nitish Kumar, the chief minister of Bihar, which borders Nepal to the north, appealed for calm and said police investigations were under way.

Singh formed Ranvir Sena in 1994 but the militia was banned by the federal and state governments in 1995. Other private upper caste militias include Diamond Sena and Sawarna Liberation Front.

But police say Ranvir Sena -- allegedly involved in nearly 30 massacres between 1995 and 2000 -- was the most formidable adversary of the ultra-left extremist groups as well as the state police, a report by the Times of India said.

Singh's violent death marks the end of a man who himself was accused of involvement in dozens of killings of lower-caste landless poor, including that of 58 Dalits -- previously called untouchables -- in December 1997.

While India's police and military continually clash with Maoist and Naxalite insurgents claiming to represent landless and poor, the right-wing militia groups of land-owners, including Ranvir Sena, have been fighting their own battles with the rebels.

The 1997 deaths of 58 Dalits in the Bihar village of Laxmanpur Bathe by Ranvir Sena members resulted in 16 of their members receiving death sentences and 10 other members getting life-in-prison sentences for their parts in the atrocities.

The court in Patna, the capital of Bihar, acquitted 19 other Ranvir Sena members.

The Laxmanpur Bathe killings were believed to be a revenge attack by Ranvir Sena on suspected Maoists and their sympathizers for a brutal attack by the rebels in Bara in 1992. In that incident, 37 upper-caste men were slaughtered allegedly by Maoists, a Times of Indian report said.

Singh -- often described as looking like a tall thin school-teacher -- held a political science degree from Patna University.

Police arrested him in Patna in 2002 and he had been held in jail pending trials. He was accused in 22 criminal cases, including massacres involving the killing of 277 persons across the state but got bail in July, a report by India Today said.

India Today said one of Singh's favorite sayings was, "Do I look like a mass murderer?"