Police in Leicester say they fear people who use knives at work end up carrying them around the city.
It comes after officers used the latest metal detector arch operation in the Humberstone Gate area on Thursday.
One craft knife was confiscated after a three-hour period during which 304 people walked through the arch.
Pc Jon Webb said sometimes people caught with knives claimed they used them as part of their jobs.
He said: "These operations are generally positive and a lot of the time the knives we confiscate are from people who claim to use them at work.
"We understand that a lot of people do use knives at work, but it's so important that they stay there. Carrying a knife is a real danger, it can be used against you and can land you serious trouble with the police which could affect your future."
The portable airport-style devices, which people walk through, are used to detect weapons and other metal objects.