Thames Valley Police have confirmed that officers are investigating the reported rape of a woman in her 20s at Hollow Way Recreation Ground in Cowley, Oxford.
The incident occurred at about 10pm on Sunday, November 8, near the junction of Horspath Road and Rupert Road.
Police attended the recreation ground late on Sunday and blocked all entrances.
The site remained closed on Monday, November 9, with officers and police vehicles at the scene.
Uniformed personnel, vehicles and plain-clothes detectives were present throughout Monday, while a police scene was maintained and neighbourhood officers patrolled nearby.
The investigation is at an early stage, with officers conducting fast-time enquiries to establish what happened.
The woman is receiving support from specially trained officers.
People living close to the park said they felt anxious and concerned after waking on Monday morning and finding it had been sealed by police.
Others reported hearing fireworks in and around the grounds late on Sunday evening, before they understood the reason for the police presence.
Officers are seeking anyone who was at Hollow Way or on nearby roads at around 10pm and witnessed anything suspicious.
They have also requested CCTV, doorbell, dash-cam and mobile phone footage recorded in the area at that time.
Information can be provided to police by quoting investigation reference 43260585129.