A United Nations privacy expert has criticised South Wales Police's use of facial recognition technology.
Wales's largest police force has used high-tech cameras mounted on a vehicle at major events - including the Biggest Weekend in Swansea.
South Wales Police, which has been asked to comment, has previously said technology is used responsibly and is a useful tool.
But UN rapporteur Joseph Cannataci described the usage as "chilling".
He made the comments at the end of a two-week visit in London, during which he met officials from GCHQ, MI5 and MI6.
"I find it difficult to see how the deployment of a technology that would potentially allow the identification of each single participant in a peaceful demonstration could possible pass the test of necessity and proportionality," he said.
South Wales Police first piloted the technology last year in Cardiff for the Champions League Final in a UK first.
However, figures showed more than 2,000 people were wrongly identified as possible criminals by facial scanning technology at the Real Madrid-Juventus game.
Of the 2,470 potential matches with custody pictures 92%, or 2,297, were wrong.
Chief Constable Matt Jukes later said officers "did not take action" and no one was wrongly arrested.
However, he justified its usage at at crowded events to protect people due to threat of terrorism and said it had become increasingly accurate due to advancements in technology.