The accuracy of police facial recognition systems has been criticised by a UK privacy group.
Two forces have been testing facial recognition cameras at public events in an effort to catch wanted criminals.
Big Brother Watch said its investigation showed the technology was "dangerous and inaccurate" as it had wrongly flagged up a "staggering" number of innocent people as suspects.
But police have defended its use and say additional safeguards are in place.
Police facial recognition cameras have been trialled at events such as football matches, festivals and parades.
High-definition cameras detect all the faces in a crowd and compare them with existing police photographs, such as mugshots from previous arrests.
Any potential matches are flagged for a police officer to investigate further.
Big Brother Watch submitted freedom of information requests to every police force in the UK.
Two police forces acknowledged they were currently testing facial recognition cameras.
Big Brother Watch said their systems had wrongly flagged thousands of innocent people.
The Metropolitan Police used facial recognition at London's Notting Hill carnival in 2016 and 2017 and at a Remembrance Sunday event.
Its system incorrectly flagged 102 people as potential suspects and led to no arrests.
In figures given to Big Brother Watch, South Wales Police said its technology had made 2,685 "matches" between May 2017 and March 2018 - but 2,451 were false alarms.
Big Brother Watch also raised concerns that photos of any "false alarms" were sometimes kept by police for weeks.
"Automated facial recognition technology is currently used by UK police forces without a clear legal basis, oversight or governmental strategy," the group said.
Leicestershire Police tested facial recognition in 2015, but is no longer using it at events.
How have the police forces responded? South Wales Police has defended its use of facial recognition software and says the system has improved with time.
The Metropolitan Police said it was testing facial recognition to see whether it could "assist police in identifying known offenders in large events, in order to protect the wider public".
Big Brother Watch wants police to stop using facial recognition technology. It has also called on the government to make sure that the police do not keep the photos of innocent people, reports BBC.