From today, anyone taking a photograph of a police officer could be deemed to have committed a criminal offence.
That is because of a new law - Section 76 of the Counter Terrorism Act - which has come into force.
It permits the arrest of anyone found "eliciting, publishing or communicating information" relating to members of the armed forces, intelligence services and police officers, which is "likely to be useful to a person committing or preparing an act of terrorism".
That means anyone taking a picture of one of those people could face a fine or a prison sentence of up to 10 years, if a link to terrorism is proved.
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The law has angered photographers, both professional and amateur, who fear it could exacerbate the harassment they already sometimes face.
On Monday, a group is gathering outside New Scotland Yard for a "mass picture-taking session" in protest.
The event is organised by the National Union of Journalists. It insists the right to take pictures in public places is "a precious freedom" that must be safeguarded.