The boy, Abdul Khalid, had gone to play in the park along with his friends last evening when he fell in the pit filled with rain water, the police said.
His friends got scared and soon left the place. It was only when Khalid's parents quizzed his friends about his whereabouts that they revealed the incident.
The pit, that was nine to ten feet deep, had been dug to dump leaves for decomposition but it was not covered and rainwater had accumulated inside, said a senior police officer.
The body has been sent for post-mortem and a case is being registered at the Vijay Vihar police station.
Police said there was no signboard near the pit indicating that it was an "out-of-bound" area. There was no fencing around the pit to indicate that it was a dangerous area.
Police will be quizzing the caretaker of the park to get more details.
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