The force had put out a statement saying “anyone who declines to be scanned will not necessarily be viewed as suspicious”. However, witnesses said several people were stopped after covering their faces or pulling up hoods.
Campaign group Big Brother Watch said one man had seen placards warning members of the public that automatic facial recognition cameras were filming them from a parked police van.
“He simply pulled up the top of his jumper over the bottom of his face, put his head down and walked past,” said director Silkie Carlo.
“There was nothing suspicious about him at all … you have the right to avoid [the cameras], you have the right to cover your face. I think he was exercising his rights.”
Ms Carlo, who was monitoring Thursday’s trial in Romford, London, told The Independent she saw a plainclothed police officer follow the man before a group of officers “pulled him over to one side”.
She said they demanded to see the man’s identification, which he gave them, and became “accusatory and aggressive”.
“The guy told them to p*** off and then they gave him the £90 public order fine for swearing,” Ms Carlo added. “He was really angry.”
A spokesperson said officers were instructed to “use their judgment” on whether to stop people who avoid cameras.