A man in his thirties was pinned down by a gang of three while walking along a cycle path in York, before being subjected to serious sexual assault.
The victim, who was walking along a path from Hob Moor to Tadcaster Road, was grabbed by two men before being assaulted by a third at 3AM on Sunday.
The cycle path was sealed off yesterday as forensic experts searched for evidence.
North Yorkshire Police were interviewing the victim yesterday to establish the exact details of the attack.
A police spokesperson said: "The victim is understandably very distressed by his ordeal and is currently being supported by specialist police officers."
House to house inquiries are being carried out in the area to uncover further information, said Detective Inspector Ian Dyer.
He added: "Even if you believe your information is irrelevant, please contact us, as even the smallest piece of information can help us build a picture of the events which led up to this assault."
The annual Jane Tomlinson Run For All 10km Race, which took place yesterday, close to the site of the attack, saw no major disruption,
In is understood that police were in discussions with the organisers but the 6000 competitors were allowed to continue as planned.
Anyone with any information can call North Yorkshire Police on 0845 6060247 or Crimestoppers anonymously on 0800 555111.