From an extraordinary account of a walk around central London:

Suspicion is a global variable. Once triggered it bubbles upward through the entire system. Walking down Park Lane, I was accosted by a man in a suit who demanded to know what I was doing. He took out his mobile phone, pointed it at my face, told me he was going to “circulate my description”.
Shortly afterwards, a colleague of his physically restrained me and called the police. Both men worked at the Grosvenor House Hotel, whose cameras were among those which had been trained on me as I walked, and so are included in my documentation.

When they arrived, the police officers explained that carrying a camera in the vicinity of Central London was grounds for suspicion. I might be a terrorist who posed a threat to the good citizens of London – my own city. Equally I might be casing the joint for some future crime, studying its defences in order to circumvent them.

Carrying a camera thus justified the suspicion of the security guards who stopped me and performed a citizen’s arrest, detaining me until the arrival of the police. This suspicion in turn justified the actions of the police, who threatened me with arrest if I did not identify myself and explain my actions. For carrying a camera, I was told, I could be taken to the station and charged with “Going Equipped”, a provision of the 1968 Theft Act which determines the imprisonment for up to three years of anyone carrying equipment which may be used to commit a burglary.