Letters encouraging recipients to take part in "Punish a Muslim Day" have reportedly been sent to addresses across the country.
Images of the A4 notes, which contain a list of violent acts alongside a number of points for performing them, have been widely shared online. They have prompted a chorus of condemnation, with campaigners and MPs describing them as "disturbing".
Counter-terrorism police are treating the letters as a possible hate crime. Tell Mama UK, which monitors anti-Muslim activity, said it had received reports of people in Bradford, Leicester, London, Cardiff and Sheffield getting the letters.
Director Iman Atta said: "This has caused quite a lot of fear within the community. They are asking if they are safe, if their children are safe to play outdoors. We have told them to keep calm and to phone the police if they receive one of these letters."
