The incident took place at around 2 PM, police spokesperson Mrityunjay Mishra said. The advocate, identified as Mohd Nabi, has been admitted to a hospital and is condition is said to be critical.
Mishra could not confirm the identity of the attacker, though eyewitnesses claimed the assailant was policeman Shailendra Singh with whom the injured advocate was said to have "a long-standing tiff".
Following the incident, the lawyers went on a rampage by hurling stones at police and administrative officials and vandalising vehicles and public property.
Further details are awaited.