Head Constable Sanjay received a gun shot wound in his left thigh in the shooting incident at around 11 am, police said.
Police said it could have been a case of accidental firing but constable Suraj Pal, who allegedly shot at Sanjay, has been detained for questioning to ascertain how the incident occurred.
Sanjay, posted in the 3rd Battalion of the Delhi Armed Police, was shot at inside the duty change room near the PMO in South Block, creating panic in the high security area.
Sanjay was rushed to Ram Manohar Lohia Hospital, where his condition is stated to be out of danger, they said.
"Only one shot was fired. We are investigating the matter," said DCP (New Delhi District) Anand Mohan.