The lawyer Sanjeev Kumar had visited Barbigha police station in connection with a case during which he had an argument with the sub-inspector, who thrashed him and wounded him in his head, sub-divisional police officer (SDPO) Amit Saran said.
Kumar was then admitted to Barbigha referral hospital.
Following protests by lawyers against the incident, the SDPO himself probed the incident and suspended sub-inspector Sitaram Singh.
An FIR has also been registered against Singh in the case, the SDPO said.