The gruesome discovery was made on a narrow pathway in the village of New Waltham near Grimsby, Lincs, at 6.45am on Thursday.
Humberside Police rushed to the scene and cordoned off the walkway, which is used by hundreds of schoolchildren every day.
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Detective Chief Inspector Craig Nicholson confirmed the body part discovered is believed to be a "lower limb".
He said: "Officers are currently on the path near to the old railway track at Louth Road, New Waltham.
"A report was received at around 6.45am today, Thursday 25 April, of a discovery of what was believed to be a human body part.
"We are in the very early stages of the investigation however it is suspected to be a lower limb.
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"Anyone with any information is asked to call our non-emergency number 101 quoting log 85 of 25/04/19 or Crimestoppers anonymously on 0800 555 111."
The discovery was made on a trail known as the New Waltham Peaks Parkway Path, which runs alongside Waltham Academy.