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This year they are out in greater force than ever, with 70,000 police officers on fashion duty in Tehran alone.
The annual crackdown has traditionally targeted women, but men are being included this year – they have been banned from sporting neck chains and their hairstyles are also under police scrutiny.
Permissible "Islamic" haircuts for men allow a dab of gel, but pony-tails and a spiked style known locally as the "rooster" are taboo.
Even so, Iranian men have it easier than the women. Men do not have to cover themselves head to toe and can wear short-sleeved shirts, although shorts are still outlawed.
Men flouting the dress code are usually hauled off to the barber or home to change.
Women caught flashing a bit of ankle, wearing a dash of make-up or pushing back their mandatory headscarves too far get a lecture on Islamic clothing and values.