Police said the incident took place on Monday when the girl was playing on the porch of her house.
The boy, who lived in a neighbouring house, came to her and took her to a secluded under-construction building and sexually assaulted her, police said.
Police said the girl later went to her house and narrated her ordeal to her parents.
A neighbour had also seen the act and raised an alarm. Her mother then approached the nearest police station and filed a complaint.
Primary investigation revealed that the incident took place in the afternoon and the FIR was filed in the evening.
Jaipur police then sent a team and examined the secne. Police said the boy was detained on Tuesday morning and was booked under different sections of the IPC and POCSO (Protection of Children from Sexual Offences) Act. Police said an FIR has been filed following the complaint and the investigation has been handed over to an additional DCP (ADCP)-rank official because of the sensitive nature of the case.
