An attacker put a jumper over a 15-year-old girl's head before raping her in dense woodland as she walked from school.
The girl was on her way to a relative's home when the man grabbed her from behind in Hampden Park, Eastbourne.
She was pulled off a footpath leading to Roseberry Avenue and then dragged into the woods.
The rapist subjected the teenager to what police said was a horrific ordeal.
The victim, wearing her school uniform, was left in the woods while the man escaped with her red Nokia 8310 mobile phone.
The attack happened at 3.30pm on Wednesday and several people who saw the girl stagger from thepark afterwards asked if she was all right.
Detectives are urgently trying to trace these people along with anyone else who may have seen a man following a lone girl in Hampden Park.
The victim, from Eastbourne, is being comforted by specialist police counsellors.
However, she has been unable to give a description of her attacker because of the blue jumper placed over her head.
House-to-house inquiries were being carried out in the area.
Fingertip searches were also being conducted yesterday within the sealed-off area, close to tennis courts, a duck pond and play and sports facilities.
Detective Inspector Martin Sapwell, leading the investigation, told The Argus: "This was a horrific attack which happened in broad daylight.
"It has left the girl extremely distressed and we are appealing as a matter of urgency for any help in catching the man responsible.
"She was aware someone was behind her. He then placed a blue jumper over her head and pulled her into woodland where he sexually assaulted and raped her.
"He had a conversation with her but we aren't prepared to disclose the contents of that.
"The whole park is used extensively and would have been quite busy during this time.
"We know several people spoke to her and we want to trace them."
A police cordon remained in place last night at the Roseberry Avenue entrance to the footpath as inquiries continued.
Anyone with information should call East Downs CID on 0845 6070999 or Crimestoppers on 0800 555111.
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