Patna, Jun 1 (IANS): An indefinite curfew was imposed in Ara town and red alert was sounded in Bihar's four districts Friday, after unidentified gunmen shot dead Brahmeshwar Singh 'Mukhiya', chief of the outlawed Ranvir Sena, police said.

Singh, in his early 70s, was killed early Friday in Ara, the district headquarters of Bhojpur, triggering violent protests by his supporters and sympathisers who constituted the upper-caste militia, according to police.

"The indefinite curfew has been imposed in view of a large-scale arson in the town," a police official in Ara, about 60 km from here, told IANS over phone.

According to officials at police headquarters in Patna, a red alert has been sounded in Jehanabad, Arwal, Aurangabad and Gaya districts. These four districts were the stronghold of Ranvir Sena and its rivals - outlawed Maoist groups and the Communist Party of India (Marxist Lenenist) - till early 2000.

"Singh was shot dead by unidentified gunmen when he was on a morning walk near his residence," a police official said.

Soon after the news of Singh's killing spread, his angry supporters attacked police vehicles, dairy vehicles and government offices in Ara. The circuit house in the town was set on fire, the official said.

Top district officials, including the superintendent of police, who reached the spot to assess and check the situation, were chased away by the protestors, the official added.

The supporters did not allow police to take Singh's body for an autopsy and blocked roads, demanding arrest of the culprits behind his killing, the official said.

The district administration has deployed security forces in the town.