Circle Officer Praveen Ranjan said that four persons had been detained in connection with the incident that occurred on Friday night and were being quizzed.
Police also rejected allegations of delayed action.
"After the girl's father made a call at 100 number, the police reached the spot within 10 minutes. The SHO was informed, and he reached there immediately," Superintendent of Police Alankrita Singh said, adding that victim got timely medical attention due to the officers.
The girl's father, however, said that they found her at 11.45 PM and the police came several hours later. She was admitted in hospital only at 7 AM yesterday.
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Recalling the horror, a relative of the girl said she and her sister were sleeping outside the house when the incident occurred. "I told my husband to bring them inside as it had started drizzling, but he said that the girl was not there. We frantically looked for her in the house and outside. Later, we found her in a field," he said.
Angered by the incident, villagers had blocked the NH-24 yesterday.
The incident comes even as the Uttar Pradesh Government has come under flak over a string of rape incidents, including the gangrape of a woman and her daughter who were dragged out of their car by the accused on National Highway-91 in Bulandshahr.
Hapur is around 65 km from Delhi.