A large kitchen knife was seized by police after officers were called to Greggs in St Ives.
Officers were flagged down by staff at the bakery chain on Monday September 2.
They reported two teenagers on the roof causing disruption.
No arrests were made but, after an area search, a large kitchen knife was recovered.
A spokesperson for Cambridgeshire Constabulary said: "This certainly isn't what we want to see on our streets.
"Officers will continue to work with local residents and businesses to take dangerous weapons off our streets as they tackle knife crime across Cambridgeshire."
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