The 16-year-old girl was allegedly raped by two men Sunny and Umesh on May 12 when she was sleeping on the roof of her house in the Civil Lines area in the home district of Samajwadi Party supremo Mulayam Singh Yadav.
One of the accused Sunny was arrested on the complaint of the victim's mother, police said.
The victim's family has alleged that it was after the arrest that the family members of the accused started pressurising victim's mother to withdraw the complaint, they said.
An FIR has been registered in this regard against Sunny's father Basant Yadav and other members of the family, Munesh and Rahul, in the Civil Lines police station.
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Superintendent of Police Rishipal Singh said while Basant had been arrested, search was on to nab others.
"We are providing full security to the victim and her family," he said.
The condition of the victim's mother, who has been admitted to a hospital here, is stated to be critical.