The bespectacled, beret-wearing shooter is a member of the paramilitary CISF or Central Industrial Security Force that helps guard the metro.
The incident took place on Sunday night at the Azadpur station in Central Delhi. Two men were being fined for inciting a fight; their friends showed up and then assaulted a constable before grabbing the collar of CISF Assistant Sub-Inspector Mohammed Umar, who then fired into the air.
Metro trains ran uninterrupted despite the incident, said officials.
Senior CISF officials and Delhi Police are looking at the CCTV video of the incident to determine who should be penalized. Sources said a 17-year-old -- who allegedly took part in the brawl -- has been arrested. Two others seen in the video have been identified. A search is on for the rest.