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The scanning of more than 55,000 faces at an annual horse fair led to one arrest.
Cumbria Police used live facial recognition (LFR) technology at the Appleby Horse Fair for the first time this year.
It said it had scanned the faces of 55,926 people over the four-day event in June which led to three people being matched to its people of interest watchlist. One person was subsequently arrested on suspicion of breach of bail, the force said.
Campaigners Big Brother Watch said the technology was "intrusive". The Cumbria force said it believed many on the watchlist had stayed away from the fair over fears the technology would catch them.
The force said the fair, which is one of the biggest events in the UK's Gypsy, Roma and traveller calendar, was its largest annual police operation.
It said LFR was just one tool it had used to ensure this year's fair had been one of the safest in many years with no significant violent incidents.
Cumbria Police said it arrested 69 people at the fair this year, compared to 80 last year.
'Digital police line-up'
The force said it had used the facial recognition technology to find people on its custom-made watchlist which varied between 1,798 and 1,826 people during event.
The list included those wanted by police forces across the country and missing persons.
It said the list had varied for many reasons including adding people to it who had been arrested during the fair.
The force said faces of those not on the watchlist who had been scanned, were deleted within an average of 0.2 seconds.
It said it had used the Asset Recovery Incentivisation Scheme - money seized from criminals - to pay the £11,500 cost of the technology.
Big Brother Watch said the use of the cameras at the fair was "concerning".
"Communities should not be made to feel they are being singled out for mass suspicion," a spokesperson said.
"It is even more outrageous considering they scanned more than 50,000 faces and made only a single arrest."
It called on the police to stop wasting time and resources on "headline-grabbing surveillance tools" which put thousands of people into a "digital police line-up".
As of March, the technology is being used by 13 out of 43 police forces in England and Wales.
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