Taking a strong note of the incident, police authorities have ordered a departmental probe into the matter.
The incident occurred yesterday when Head Constable Nepal Singh allowed an undertrial prisoner to "sit and talk" to his family members at Sadar Bazar police station.
Constable Surendra Singh objected to it, saying it was in violation of rules.
This led to an argument between two policemen. Flying into a rage, Nepal hit Surendra with a blunt object, injuring him seriously.
The constable was rushed to a hospital, where he is undergoing treatment.