The incident took place yesterday evening, when the victim, identified as Musa Ali, a class II student of a private school at Turkman Gate, was going to market to purchase clothes, along with his father.
"As Musa and his father was about to cross the Kalan Masjid road, a window frame from an under-construction building fell on him," said a police officer.
Everything happened so sudden that the victim's father fainted on the spot seeing his son critically injured lying in a pool of blood.
Soon, the body was sent for post-mortem, after which it was handed over to his parents.
A case of negligence was registered at the Chandni Mahal police station and the builder has been arrested, the officer said, adding it is also being checked whether the construction was in violation of rules.