The constable, identified as Satyendra S Maan, boarded the Metro at Kashmere Gate and got into a tiff with commuters.
As Maan was on duty, he was allowed to board the Metro with his official pistol after following the mandatory drill of recording the particulars of the weapon in an official register, official sources said.
During the journey, Maan, according to the sources, picked up a quarrel in the Metro train claiming his belongings has been stolen and he would shoot with his weapon.
The commuters raised an alarm and informed the Metro authorities and on their call a CISF squad intercepted the constable at Seelampur Metro station at around 1935 hours, they said.
The constable, who works in the Special Branch of Delhi Police, was handed over to the Metro police who questioned him, the sources said.