In a complaint, a woman from Nasirpur village in Karnal said she was travelling from Jind to Karnal with her daughter on Saturday evening in a bus, they said.
She alleged that as the bus was crowded, she made her daughter sit next to ASI Rampal but she later saw that he was sexually assaulting her daughter.
On raising an alarm, Rampal, who appeared to be intoxicated, was caught by the other passengers and was handed over to the police.
The girl was sent for medical examination and a case registered against the accused under section 10 (aggravated assault) of the POCSO Act following which he was arrested, Inspector Roshni Devi of Mahila Police Station said.
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