The founder of the now defunct private army of landlords was behind several bloody massacres and named in 22 cases.
Enraged supporters of Singh targeted government property, trains and vehicles, burned down vehicles and blocked highways.
The Ara administration imposed a one-day curfew and rushed in paramilitary forces. An alert was sounded across Bihar. Chief minister Nitish Kumar appealed for calm.
The murder may have a political fallout as Singh belonged to the Bhumihar caste, which is considered to be a pillar of support for the ruling JD(U) and BJP. RJD chief Lalu Prasad was quick to demand a CBI probe.
Internal police communication with security agencies blamed the murder on rivalry within the Sena or individuals with vested political interests. No arrests were made till late evening.