UK police will now use facial recognition technology to scan the faces of passersby on the street to see if they are criminals.

London's Metropolitan Police said on Friday that it would deploy standalone facial recognition cameras through parts of the capital to tackle knife and gun violence, terrorism, and other major crimes.

The force said the cameras would be signposted, and officers close by would hand out leaflets about the system.

Civil liberties campaign group Big Brother Watch vowed to fight the implementation of facial recognition and slammed it as "an enormous expansion of the surveillance state and a serious threat to civil liberties in the UK."

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The UK's largest police force will use facial recognition cameras to scan the faces of passersby to see if they are criminals, in a move that critics slammed as a massive expansion of surveillance.

In a statement on its website published Friday, London's Metropolitan Police said the system would involve using cameras to focus on "a small, targeted area to scan passers-by."

The force said cameras would be signposted, and offices involved in the operation would hand out leaflets about the activity.

The Metropolitan Police hasn't specified where in London the technology will go live, but said it would place cameras in areas where it could locate "serious offenders."

The goal is to tackle serious crime, including serious violence, gun and knife crime, child sexual exploitation. It's also intended to help with the protection of vulnerable people.

But the rollout of the technology, which is being supplied by Japanese technology firm NEC, is likely to prompt serious concerns about government surveillance and civil liberties.

Police constables Ben Sinclair and Karen Spencer pose for a photograph wearing their Metropolitan Police beat uniforms, in London, October 9, 2014. Paul Hackett/Reuters

When UK police previously trialled facial recognition tech, they fined people for covering their faces

When the Metropolitan Police first trialled facial recognition technology in January 2019, a man was reported as having been controversially fined £90 ($117) after refusing to show his face.

However, a Met Police spokesman told Business Insider that the man was not fined for refusing to show his face, but was fined for "disorder" caused at the scene.

The spokesman also confirmed that people who refuse to show their faces in areas where the tech is in operation will not be fined for doing so.

Civil liberties campaign groups have reacted angrily to the Met's decision, however.

Silkie Carlo, director of civil liberties campaign group Big Brother Watch, told Business Insider: "This decision represents an enormous expansion of the surveillance state and a serious threat to civil liberties in the UK."

She added that the Met police's earlier trial was "81% inaccurate", citing findings by an independent report.

She added: "This is a breath-taking assault on our rights and we will challenge it including by urgently considering next steps in our ongoing legal claim against the Met and the Home Secretary.

"This move instantly stains the new Government's human rights record and we urge an immediate reconsideration."

Speaking about the decision, the Met's assistant commissioner Nick Ephgrave, said: "This is an important development for the Met and one which is vital in assisting us in bearing down on violence. As a modern police force, I believe that we have a duty to use new technologies to keep people safe in London."