The Metropolitan police have been accused of defying the warnings of its own watchdogs by beginning operational use of facial recognition CCTV, despite a scathing assessment of its effectiveness from the expert hired to scrutinise its trials.

On Tuesday, thousands of shoppers in east London were scanned by van-mounted cameras pointed at the doors of the Stratford Centre retail complex, in what a senior police officer at the scene described as an “intelligence-led” deployment.

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Commander Mark McEwen, the Met’s lead on crime prevention, said Stratford had been chosen because it had been the scene of “public space violence”, and that there was support from the community for the police to use “whatever tactic we can to deal with violence”.

He said the face of each passerby was being scanned and checked against a watchlist of 5,000 biometric profiles of “people who are wanted for serious criminality, such as grievous bodily harm. There may be warrants [for their arrest], or they may be wanted by the courts, or they may be wanted by us for investigations.”

When the Guardian arrived, about two dozen uniformed officers stood in the area around the dark blue van on which the cameras were mounted. Some stood in small groups, peering at the smartphones to which alerts of any positive match would be sent. No one was stopped.

Signs around the van announced that “live facial recognition [LFR]” was in use, and that “there is no legal requirement for you to pass through the LFR system”.

However, Siân Berry, the co-leader of the Green party, visiting the operation, noted that by the time the sign was visible to passersby they were already in the cameras’ field of view.

“In theory I should be able to avoid having my face scanned,” she said. Some people covered their faces after seeing the sign.

“The police have gone ahead and used [facial recognition] in defiance of some serious warnings that have been issued by people like the information commissioner, the surveillance camera commissioner and the biometric commissioner,” Berry said.

Silkie Carlo, the director of the privacy rights group Big Brother Watch (BBW), stood by the van for much of the day with a placard saying “Stop facial recognition”.

She described the police operation as a tipping point. “If we let this slide, this is going to be the beginning of something much worse. If they are successful in rolling this out and the legal challenges don’t work we will see this on CCTV networks pretty soon.”

Facebook Twitter Pinterest Silkie Carlo protesting next to the police van. Photograph: Damien Gayle/The Guardian

Carlo cited research by BBW that showed 93% of those stopped during all 10 public Met trials were wrongly identified. Last month, Prof Pete Fussey – an expert on surveillance from Essex University who conducted the only independent review of the Met’s trials on behalf on the force – found it was verifiably accurate in just 19% of cases.

McEwen contested the statistics. “We are content that through the tests that we have carried out that false positives are one in a thousand,” he said, referring instead to the number of false positives among the total number of people who had walked past the cameras in the trials.

The Met did not answer questions about how Tuesday’s operational use of facial recognition technology differed from previous trial uses.