Police said the blast took place when a prison van arrived at the premises carrying under-trials for production before the court.
Zonal Inspector General (IG) Amarendra Kumar Ambedkar said the incident occurred at noon.
"It is not clear whether it was a terrorist incident and investigations have started," Ambedkar, who is based in Patna, said.
The Bhojpur district superintendent of police and other administrative officials are at the spot.
The injured have been rushed to hospital.