Use of real-time facial recognition is less common. NEC, a Japanese company that makes biometric and facial recognition services, sold London the technology now being adopted. Other buyers of its real-time facial recognition technology include Surat, a city of about five million people in India, and the country of Georgia, according to the company’s website.

The technology is also used every few weeks in Cardiff, the capital of Wales, often at big events like rugby matches or a concert for the heavy metal band Slipknot this past week. As of September, police in Wales say, the technology had helped in the arrests of 58 people who had been wanted.

Representatives at NEC did not respond to a request for comment.

According to researchers at Georgetown University, several American cities have piloted the live facial recognition systems, often with mixed results. In Orlando, Fla., a pilot program that ended last year tried to match faces going past several cameras against individuals on a watch list. In Detroit, police purchased a face-identification system as part of a crime-prevention program. Last year, The Wall Street Journal reported on the failures of a New York pilot program to spot people as they drove past bridges and tunnels.

Use of facial recognition technology in the United States has generated a backlash. San Francisco, Oakland and Berkeley in California, along with Somerville and Brookline in Massachusetts, have banned its use.

Privacy groups criticized London’s decision and vowed to take legal action to try to stop the deployment of the software.

“This decision represents an enormous expansion of the surveillance state and a serious threat to civil liberties in the U.K.,” said Silkie Carlo, director of Big Brother Watch, a London-based group that has been fighting the use of facial recognition. “This is a breathtaking assault on our rights, and we will challenge it.”

Last year, a British judge said that police departments could use the technology without violating privacy or human rights, a case that is under appeal. The government’s top privacy regulator has raised concerns about the use of the technology, as did an independent report of a trial use by the Metropolitan Police.