Analysis A new ad campaign by the Metropolitan Police warns the public to be on the alert for strangers who look at them in a funny way, and to check the contents of their neighbours’ bins. Suggestions that the campaign may be a tad alarmist have been dismissed, the police arguing they are only doing what is necessary to protect the public in dangerous times.

Meanwhile, lawyers already concerned by what they see as police overreaction and exaggeration of threat have warned that this campaign may represent a much more sinister attempt to extend police powers into places where Parliament has hitherto been unwilling to allow them.

First the campaign. About a week ago, a reader alerted us to a Met campaign warning Londoners to be on the lookout for suspicious people and circumstances: chemical containers left lying in a wheelie bin, or people staring "suspiciously" at CCTV cameras.

This is in line with the overall Met message of "If you suspect it, report it", which urges Londoners to trust their instincts and to report any activity they believe suspicious, and posters such as the anti-photography one, which suggests that people taking photos may be suspicious.

The official police line is that this is no more than sensible precaution. A spokeswoman for the Met said: "Acts of terrorism involve planning stages." She went on to explain that the Met are merely "giving examples of suspicious activity which could be linked to terrorism that people should look out for".

She also cited instances in which behaviour of the sort warned against formed part of the case against individuals later convicted of terrorist offences. Following Operation Rhyme, Dhiren Barot was sentenced to 30 years’ jail in 2006 for conspiracy to murder: he had previously filmed security measures - including CCTV - in various US cities.

In the UK, 2008's Operation Vivace led to the jailing for 40 years of Muktar Said Ibrahim and a number of co-defendants: they had used hydrogen peroxide in bombs which were detonated on the London transport network two weeks after the 7/7 bombings. Hundreds of litres of hydrogen peroxide were purchased by the defendants, and bottles were recovered from Ibrahim’s flat and the bin area at the flats after they were spotted by a caretaker.

One problem with this debate is that it has consequences well beyond the psychological: because in law, police powers depend directly on the reasonableness – or otherwise – of the suspicions they have in specific situations.

Under s1 of the Police and Criminal Evidence Act 1984, a police officer may stop an individual to look for stolen or prohibited items where he has reasonable cause for supsicion. s60 of that Act provides a broader power to search where a senior officer has declared there to be a specific risk of violence. S43 of the Terror Act 2000 gives an officer the power to stop and search anyone he reasonably suspects to be a terrorist, for items that may help prove that fact.