Civil liberties and race relations groups have demanded the Metropolitan Police abandons plans to use facial recognition technology at this month's Notting Hill Carnival.
The use of the technology could be discriminatory they claim.
The groups also warn scanning the faces of carnival-goers has no basis in law.
The Met, which used the technology for the first time at last year's carnival, said it would continue to trial the system at this year's event.
Liberty, Privacy International, StopWatch and Black Lives Matter have all written to the Met warning using the technology could lead to discriminatory policing and that it was a gross violation of carnival-goers' privacy.
The technology uses biometric software to scan the faces of passers-by, creating maps of facial characteristics that are as unique as fingerprints.
Liberty claims the scans will be compared to images on an unknown database, the details of which have not been disclosed by the Metropolitan Police.
But the Met said the database would only contain images of individuals who were "already forbidden from attending carnival, as well as individuals wanted by police".
The letter urges Met Commissioner Cressida Dick to scrap the plan.
Martha Spurrier, director of Liberty, said: "There are no laws, no rules and no oversight for facial recognition technology - not to mention the serious concerns about its accuracy.
"It is a shady enterprise neither our MPs nor the public have consented to or know enough about.
"There are significant doubts as to whether deploying this technology in public spaces can ever be lawful - especially without proper Parliamentary debate.
"The Met must urgently abandon its plans so that the thousands of people hoping to enjoy the carnival weekend know their police force will protect their human rights."
A spokesman for the Met Police said only those images which came up as a match to a wanted offender would be retained by police.
"The deployment of the cameras is to test the technology and was implemented following close liaison with the Information Commissioner and Big Brother Watch," he said.
"Once the trial is complete, we will be analysing the results and holding a public consultation."
The Met added while it was possible the faces of members of the public in the background of a positive identification might also feature on the database, those images would "be retained for the purposes of analysis of this project only and will not be speculatively searched or disseminated for any purpose".